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In JavaScript if you try to use a variable that doesn't exist and has not been declared, then JavaScript will throw an error
var name is not definedand the script will stop execute thereafter. But If you usetypeof undeclared_variablethen it will returnundefined.
Before starting further discussion let's understand the difference between declaration and definition.
var x is a declaration because you are not defining what value it holds yet, but you are declaring its existence and the need of memory allocation.
var x; // declaring x
console.log(x); //output: undefinedvar x = 1 is both declaration and definition (also we can say we are doing initialisation), Here declaration and assignment of value happen inline for variable x, In JavaScript every variable declaration and function declaration brings to the top of its current scope in which it's declared then assignment happen in order this term is called hoisting.
A variable that is declared but not define and when we try to access it, It will result
undefined.
var x; // Declaration
if(typeof x === 'undefined') // Will return trueA variable that neither declared nor defined when we try to reference such variable then It result
not defined.
console.log(y); // Output: ReferenceError: y is not definedhttp://stackoverflow.com/questions/20822022/javascript-variable-definition-declaration
var y = 1;
if (function f() {}) {
y += typeof f;
}
console.log(y);Above code would give output
1undefined. If condition statement evaluate usingevalsoeval(function f() {})which returnfunction f() {}which is true so inside if statement code execute.typeof freturn undefined because if statement code execute at run time, so statement insideifcondition evaluated at run time.
var k = 1;
if (1) {
eval(function foo() {});
k += typeof foo;
}
console.log(k);Above code will also output 1undefined.
var k = 1;
if (1) {
function foo() {};
k += typeof foo;
}
console.log(k); // output 1functionOne of the drawback of creating a true private method in JavaScript is that they are very memory inefficient because a new copy of the method would be created for each instance.
var Employee = function (name, company, salary) {
this.name = name || ""; //Public attribute default value is null
this.company = company || ""; //Public attribute default value is null
this.salary = salary || 5000; //Public attribute default value is null
// Private method
var increaseSalary = function () {
this.salary = this.salary + 1000;
};
// Public method
this.dispalyIncreasedSalary = function() {
increaseSalary();
console.log(this.salary);
};
};
// Create Employee class object
var emp1 = new Employee("John","Pluto",3000);
// Create Employee class object
var emp2 = new Employee("Merry","Pluto",2000);
// Create Employee class object
var emp3 = new Employee("Ren","Pluto",2500);Here each instance variable emp1, emp2, emp3 has own copy of increaseSalary private method.
So as recommendation don't go for a private method unless it's necessary.
A closure is a function defined inside another function (called parent function) and has access to the variable which is declared and defined in parent function scope.
The closure has access to variable in three scopes:
- Variable declared in his own scope
- Variable declared in parent function scope
- Variable declared in global namespace
var globalVar = "abc";
// Parent self invoking function
(function outerFunction (outerArg) { // begin of scope outerFunction
// Variable declared in outerFunction function scope
var outerFuncVar = 'x';
// Closure self-invoking function
(function innerFunction (innerArg) { // begin of scope innerFunction
// variable declared in innerFunction function scope
var innerFuncVar = "y";
console.log(
"outerArg = " + outerArg + "\n" +
"outerFuncVar = " + outerFuncVar + "\n" +
"innerArg = " + innerArg + "\n" +
"innerFuncVar = " + innerFuncVar + "\n" +
"globalVar = " + globalVar);
// end of scope innerFunction
})(5); // Pass 5 as parameter
// end of scope outerFunction
})(7); // Pass 7 as parameterinnerFunction is closure which is defined inside outerFunction and has access to all variable which is declared and defined in outerFunction scope. In addition to this function defined inside function as closure has access to variable which is declared in global namespace.
Output of above code would be:
outerArg = 7
outerFuncVar = x
innerArg = 5
innerFuncVar = y
globalVar = abcconsole.log(mul(2)(3)(4)); // output : 24
console.log(mul(4)(3)(4)); // output : 48Below is code followed by an explanation how it works:
function mul (x) {
return function (y) { // anonymous function
return function (z) { // anonymous function
return x * y * z;
};
};
}Here mul function accept the first argument and return anonymous function which take the second parameter and return anonymous function which take the third parameter and return multiplication of arguments which is being passed in successive
In Javascript function defined inside has access to outer function variable and function is the first class object so it can be returned by function as well and passed as argument in another function.
- A function is an instance of the Object type
- A function can have properties and has a link back to its constructor method
- Function can be stored as variable
- Function can be pass as a parameter to another function
- Function can be returned from function
For instance:
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f'];How can we empty above array?
There are a couple of ways by which we can empty an array, So let's discuss all the possible way by which we can empty an array.
arrayList = [];Above code will set the variable arrayList to a new empty array. This is recommended if you don't have references to the original array arrayList anywhere else because It will actually create a new empty array. You should be careful with this way of empty the array, because if you have referenced this array from another variable, then the original reference array will remain unchanged, Only use this way if you have only referenced the array by its original variable arrayList.
For Instance:
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList; // Referenced arrayList by another variable
arrayList = []; // Empty the array
console.log(anotherArrayList); // Output ['a', 'b', 'c', 'd', 'e', 'f']arrayList.length = 0;Above code will clear the existing array by setting its length to 0. This way of empty the array also update all the reference variable which pointing to the original array. This way of empty the array is useful when you want to update all the another reference variable which pointing to arrayList.
For Instance:
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList; // Referenced arrayList by another variable
arrayList.length = 0; // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []arrayList.splice(0, arrayList.length);Above implementation will also work perfectly. This way of empty the array will also update all the references of the original array.
var arrayList = ['a', 'b', 'c', 'd', 'e', 'f']; // Created array
var anotherArrayList = arrayList; // Referenced arrayList by another variable
arrayList.splice(0, arrayList.length); // Empty the array by setting length to 0
console.log(anotherArrayList); // Output []while(arrayList.length) {
arrayList.pop();
}Above implementation can also empty the array. But not recommended to use often.
The best way to find whether an object is instance of a particular class or not using
toStringmethod fromObject.prototype
var arrayList = [1 , 2, 3];One of the best use cases of type checking of an object is when we do method overloading in JavaScript. For understanding this let say we have a method called greet which take one single string and also a list of string, so making our greet method workable in both situation we need to know what kind of parameter is being passed, is it single value or list of value?
function greet(param) {
if() {
// here have to check whether param is array or not
}
else {
}
}However, in above implementation it might not necessary to check type for array, we can check for single value string and put array logic code in else block, let see below code for the same.
function greet(param) {
if(typeof param === 'string') {
}
else {
// If param is of type array then this block of code would execute
}
}Now it's fine we can go with above two implementations, but when we have a situation like a parameter can be single value, array, and object type then we will be in trouble.
Coming back to checking type of object, As we mentioned that we can use Object.prototype.toString
if(Object.prototype.toString.call(arrayList) === '[object Array]') {
console.log('Array!');
}If you are using jQuery then you can also used jQuery isArray method:
if($.isArray(arrayList)) {
console.log('Array');
} else {
console.log('Not an array');
}FYI jQuery uses Object.prototype.toString.call internally to check whether an object is an array or not.
In modern browser, you can also use:
Array.isArray(arrayList);Array.isArray is supported by Chrome 5, Firefox 4.0, IE 9, Opera 10.5 and Safari 5
var output = (function(x) {
delete x;
return x;
})(0);
console.log(output);Above code will output
0as output.deleteoperator is used to delete a property from an object. Herexis not an object it's local variable.deleteoperator doesn't affect local variable.
var x = 1;
var output = (function() {
delete x;
return x;
})();
console.log(output);Above code will output
1as output.deleteoperator is used to delete property from object. Herexis not an object it's global variable of typenumber.
var x = { foo : 1};
var output = (function() {
delete x.foo;
return x.foo;
})();
console.log(output);Above code will output
undefinedas output.deleteoperator is used to delete a property from an object. Herexis an object which has foo as a property and from self-invoking function we are deleting foo property of objectxand after deletion we are trying to reference deleted propertyfoowhich resultundefined.
var Employee = {
company: 'xyz'
}
var emp1 = Object.create(Employee);
delete emp1.company
console.log(emp1.company);Above code will output
xyzas output. Hereemp1object got company as prototype property. delete operator doesn't delete prototype property.
emp1 object doesn't have company as its own property. you can test it console.log(emp1.hasOwnProperty('company')); //output : false However, we can delete company property directly from Employee object using delete Employee.company or we can also delete from emp1 object using __proto__ property delete emp1.__proto__.company.
var trees = ["redwood", "bay", "cedar", "oak", "maple"];
delete trees[3];when you run above code and do console.log(trees); in chrome developer console then you will get ["redwood", "bay", "cedar", undefined × 1, "maple"] and when you run above code in Firefox browser console then you will get ["redwood", "bay", "cedar", undefined, "maple"] so from these it's cleared that chrome has its own way of displaying uninitialized index in array. But when you check trees[3] === undefined in both of the browser you will get similar output as true.
Note: Please remember you need not check for uninitialized index of array in trees[3] === 'undefined × 1' it will give an error, Because 'undefined × 1' this is just way of displaying uninitialized index of array in chrome.
var trees = ["xyz", "xxxx", "test", "ryan", "apple"];
delete trees[3];
console.log(trees.length);Above code will output
5as output. When we useddeleteoperator for deleting an array element then, the array length is not affected from this. This holds even if you deleted all the element of array usingdeleteoperator.
So when delete operator removes an array element that deleted element is not longer present in array. In place of value at deleted index undefined x 1 in chrome and undefined is placed at the index. If you do console.log(trees) output ["xyz", "xxxx", "test", undefined × 1, "apple"] in Chrome and in Firefox ["xyz", "xxxx", "test", undefined, "apple"].
var bar = true;
console.log(bar + 0);
console.log(bar + "xyz");
console.log(bar + true);
console.log(bar + false); Above code will output
1, "truexyz", 2, 1as output. General guideline for addition of operator:
- Number + Number -> Addition
- Boolean + Number -> Addition
- Number + String -> Concatenation
- String + Boolean -> Concatenation
- String + String -> Concatenation
var z = 1, y = z = typeof y;
console.log(y);Above code will output
undefinedas output. According toassociativityrule operator with the same precedence are processed based on their associativity property of operator. Here associativity of the assignment operator isRight to Leftso firsttypeof ywill evaluate first which isundefinedand assigned tozand thenywould be assigned the value of z and thenzwould be assign value1.
// NFE (Named Function Expression
var foo = function bar() { return 12; };
typeof bar();Above code will output
Reference Erroras output. For making above code work you can re-write above code as follow:
Sample 1
var bar = function() { return 12; };
typeof bar();or
Sample 2
function bar() { return 12; };
typeof bar();The function definition can have only one reference variable as a function name, In above code sample 1 bar is reference variable which is pointing to anonymous function and in sample 2 function definition is name function.
var foo = function bar() {
// foo is visible here
// bar is visible here
console.log(typeof bar()); // Work here :)
};
// foo is visible here
// bar is undefined herevar foo = function() {
// Some code
};function bar() {
// Some code
};The main difference is function foo is defined at run-time whereas function bar is defined at parse time. For understanding It in better way let see below code :
// Run-Time function declaration
<script>
foo(); // Call foo function here, It will give an error
var foo = function() {
console.log("Hi I am inside Foo");
};
</script>// Parse-Time function declaration
<script>
bar(); // Call bar function here, It will not give an Error
function bar() {
console.log("Hi I am inside Foo");
};
</script>The another advantage of first-one way of declaration that you can declare function based on certain condition for example:
<script>
if(testCondition) { // If testCondition is true then
function foo() {
console.log("inside Foo with testCondition True value");
};
} else {
function foo() {
console.log("inside Foo with testCondition false value");
};
}
</script>But If you try to run similar code in the following format, it would give an error
<script>
if(testCondition) {// If testCondition is true then
var foo = function() {
console.log("inside Foo with testCondition True value");
};
} else {
var foo = function() {
console.log("inside Foo with testCondition false value");
};
}
</script>Function Expression
var foo = fun
F438
ction foo() {
return 12;
};In JavaScript variable and functions are
hoisted. Let's take functionhoistingfirst. Basically, the JavaScript interpreter looks ahead to find all the variable declaration and hoists them to the top of the function where it's declared. For Example:
foo(); // Here foo is still undefined
var foo = function foo() {
return 12;
};Above code behind the scene look something like below code:
var foo = undefined;
foo(); // Here foo is undefined
foo = function foo() {
// Some code stuff
}var foo = undefined;
foo = function foo() {
// Some code stuff
}
foo(); // Now foo is defined herevar salary = "1000$";
(function () {
console.log("Original salary was " + salary);
var salary = "5000$";
console.log("My New Salary " + salary);
})();Above code will output:
undefined, 5000$. JavaScript has hoisting concept where newbie gets tricked. In above code, you might be expectingsalaryto retain it values from outer scope until the point thatsalarywas re-declared in the inner scope. But due tohoistingsalary value wasundefinedinstead. To understand it better have a look of the following code, heresalaryvariable is hoisted and declared at the top in function scope and while doing console.log it's resultundefinedand after that it's been redeclared and assigned5000$.
var salary = "1000$";
(function () {
var salary = undefined;
console.log("Original salary was " + salary);
salary = "5000$";
console.log("My New Salary " + salary);
})();Question 20. What is the instanceof operator in JavaScript? what would be the output of the following code?
function foo() {
return foo;
}
new foo() instanceof foo;
instanceofoperator checks the current object and return true if the object is of the specified type.
For Example:
var dog = new Animal();
dog instanceof Animal; // Output : trueHere dog instanceof Animal is true since dog inherits from Animal.prototype
var name = new String("xyz");
name instanceof String; // Output : trueHere name instanceof String is true since name inherits from String.prototype. Now let's understand the working of the following code
function foo() {
return foo;
}
new foo() instanceof foo;Here function foo is returning foo which is again pointer to function foo
function foo() {
return foo;
}
var bar = new foo();
// here bar is pointer to function foo() {return foo}.So the new foo() instanceof foo return false;
Ref Link: http://stackoverflow.com/questions/2449254/what-is-the-instanceof-operator-in-javascript
var counterArray = {
A : 3,
B : 4
};
counterArray["C"] = 1;There are no built-in function and property available to calculate length of associative array object. However, there are ways by which we can calculate the length of associative array object, In addition to this we can also extend
Objectby adding method or property on prototype for calculating length but extending object might break enumeration in various libraries or might create cross-browser issue, so it's not recommended unless it's necessary. There is various ways by which we can calculate length.
Object has keys method which can we used to calculate the length of object.
Object.keys(counterArray).length; // Output 3We can also calculate length of object by iterating through the object and by doing a count of own property of object.
function getSize(object) {
var count = 0;
for(key in object) {
// hasOwnProperty method check own property of object
if(object.hasOwnProperty(key)) count++;
}
return count;
}We can also add
lengthmethod directly onObjectsee below code.
Object.length = function() {
var count = 0;
for(key in object) {
// hasOwnProperty method check own property of object
if(object.hasOwnProperty(key)) count++;
}
return count;
}
//Get the size of any object using
console.log(Object.length(counterArray));Bonus: We can also use Underscore (recommended, as it's lightweight) to calculate object length.
If your are familiar with Object-oriented programming, More likely familiar to thinking of functions, methods, and class constructors as three separate things. But In JavaScript, these are just three different usage patterns of one single construct.
functions : The simplest usages of function call:
function helloWorld(name) {
return "hello world, " + name;
}
helloWorld("JS Geeks"); // "hello world JS Geeks"Methods in JavaScript are nothing more than object properties that reference to a function.
var obj = {
helloWorld : function() {
return "hello world, " + this.name;
},
name: 'John Carter'
}
obj.helloWorld(); // // "hello world John Carter"Notice how helloWorld refer to this properties of obj. Here it's clear or you might have already understood that this gets bound to obj. But the interesting point that we can copy a reference to the same function helloWorld in another object and get a difference answer. Let see:
var obj2 = {
helloWorld : obj.helloWorld,
name: 'John Doe'
}
obj2.helloWorld(); // "hello world John Doe"You might be wonder what exactly happens in a method call here. Here we call the expression itself determine the binding of this this, The expression obj2.helloWorld() looks up the helloWorld property of obj and calls it with receiver object obj2.
The third use of functions is as constructors. Like function and method, constructors are defined with function.
function Employee(name, age) {
this.name = name;
this.age = age;
}
var emp1 = new Employee('John Doe', 28);
emp1.name; // "John Doe"
emp1.age; // 28Unlike function calls and method calls, a constructor call new Employee('John Doe', 28) create a brand new object and passes it as the value of this, and implicitly returns the new object as its result.
The primary role of the constructor function is to initialize the object.
function User(name) {
this.name = name || "JsGeeks";
}
var person = new User("xyz")["location"] = "USA";
console.log(person);The output of above code would be USA. Here new User("xyz") creates a brand new object and created property location on that and USA has been assigned to object property location and that has been referenced by the person.
Let say new User("xyz") crated a object called foo and returned now foo["location"] would be assigned value as USA and person is referencing to foo["location"].
A function is a piece of code that is called by name and function itself not associated with any object and not defined inside any object. It can be passed data to operate on (i.e. parameter) and can optionally return data (the return value).
// Function definition
function myFunc() {
// Do some stuff;
}
// Calling function
myFunc();Here myFunc() function call is not associated with object hence not invoked through any object.
A function can be self-invoking anonymous function or named self-invoking function
// Named Self-invoking Function
(function myFunc() {
// Do some stuff;
})();
// Anonymous Self-invoking Function
(function() {
// Do some stuff;
})();In a case of named self-invoking anonymous function or anonymous self-invoking function, there is no need of call function explicitly.
A method is a piece of code that is called by name and that is associated with the object. In most respects it is identical to function call except for some key difference:
- It is implicitly passed for the object for which it was called.
- It is able to operate on data that is contained within the class (remembering that an object is an instance of a class- the class is the definition, the object is an instance of that)
- Method call is always associated with object
var methodObject = {
attribute: "xyz",
display: function () { // Method
console.log(this.attribute);
}
};
// Call method
methodObject.display();Here methodObject is an object and display is a method which is associated with methodObject.
Question 26. What is JavaScript Self-Invoking anonymous function or Self-Executing anonymous function.
A self-invoking anonymous function also called self-executing anonymous function runs immediately or automatically when we define it and self-invoking anonymous function doesn't have any name at all. Self-Invoking anonymous function syntax:
(function() {
console.log("Self-Invoking function code logic ");
})();
Output: Self-Invoking function code logicWe must have to know the fact that JavaScript functions run immediately when we put () after their names.
function display() {
console.log("Display me");
}
display(); // This will run immediately
Output: "Display me"
/*
This will not run immediately as we haven't put () after function
name, function name is use full when we want to pass it as
callback to another function or method.
*/
display;
function testCallBack(callback) {
callback (); // This will be call display method immediately if callback parameter is being set method display
}
testCallBack(display); // Here display function is being passed as callbackThe singleton pattern is the most commonly used design pattern and one that you will probably is more than any others. It provides a great way to wrap the code into a logical unit that can be accessed through a single variable. The Singleton design pattern is used when only one instance of an object is needed throughout the lifetime of an application.
In JavaScript, there is a different way to achieve singleton object than any other object oriented supported language (Java, C++). In JavaScript Singleton pattern have many uses, they can be used for NameSpacing, which reduce the number of global variables in your page (prevent from polluting global space), organizing the code in a consistent manner, which increase the readability and maintainability of your pages. There are two important points in the traditional definition of Singleton pattern:
- There should be only one instance allowed for a class and
- We should allow global point of access to that single instance
- Let me define singleton pattern in JavaScript context:
It is an object that is used to create namespace and group together a related set of methods and attributes (encapsulation) and if we allow to initiate then it can be initiated only once.
In JavaScript, we can create singleton though object literal. However, there is some another way but that I will cover in next post.
A singleton object consists of two parts: The object itself, containing the members (Both methods and attributes) within it, and global variable used to access it. The variable is global so that object can be accessed anywhere in the page, this is a key feature of the singleton pattern.
JavaScript: A Singleton as a Namespace
As I have already stated above that singleton can be used to declare Namespace in JavaScript. NameSpacing is a large part of responsible programming in JavaScript. Because everything can be overwritten, and it is very easy to wipe out variable by mistake or a function, or even a class without even knowing it. A common example which happens frequently when you are working with another team member parallel,
function findUserName(id) {
}
/* Later in the page another programmer
added code */
var findUserName = $('#user_list');
/* You are trying to call :( */
console.log(findUserName())One of the best ways to prevent accidentally overwriting variable is to namespace your code within a singleton object.
/* Using Namespace */
var MyNameSpace = {
findUserName : function(id) {},
// Other methods and attribute go here as well
}
/* Later in the page another programmer
added code */
var findUserName = $('#user_list');
/* You are trying to call and you make this time workable */
console.log(MyNameSpace.findUserName());/* Lazy Instantiation skeleton for a singleton pattern */
var MyNameSpace = {};
MyNameSpace.Singleton = (function() {
// Private attribute that holds the single instance
var singletonInstance;
// All of the normal code goes here
function constructor() {
// Private members
var privateVar1 = "Nishant";
var privateVar2 = [1,2,3,4,5];
function privateMethod1() {
// code stuff
}
function privateMethod1() {
// code stuff
}
return {
attribute1 : "Nishant",
publicMethod: function() {
alert("Nishant");// some code logic
}
}
}
return {
// public method (Global access point to Singleton object)
getInstance: function() {
//instance already exist then return
if(!singletonInstance) {
singletonInstance = constructor();
}
return singletonInstance;
}
}
})();
// getting access of publicMethod
console.log(MyNamespace.Singleton.getInstance().publicMethod());The singleton implemented above is easy to understand. The singleton class maintains a static reference to the lone singleton instance and return that reference from the static getInstance() method.
Method 1: Function Based
If we want to create several similar objects. In below code sample, Employee which is called constructor function. This is similar to classes in object oriented languages.
function Employee(fName, lName, age, salary){
this.firstName = fName;
this.lastName = lName;
this.age = age;
this.salary = salary;
}
// Creating multiple object which have similar property but diff value assigned to object property.
var employee1 = new Employee('John', 'Moto', 24, '5000$');
var employee1 = new Employee('Ryan', 'Jor', 26, '3000$');
var employee1 = new Employee('Andre', 'Salt', 26, '4000$');Method 2: Object Literal
Object Literal is best way to create an object and this is used frequently. Below is code sample for create employee object which contains property as well as method.
var employee = {
name : 'Nishant',
salary : 245678,
getName : function(){
return this.name;
}
}Below code sample is Nested Object Literal, Here address is an object inside employee object.
var employee = {
name : 'Nishant',
salary : 245678,
address : {
addressLine1 : 'BITS Pilani',
addressLine2 : 'Vidya Vihar'.
phoneNumber: {
workPhone: 7098889765,
homePhone: 1234567898
}
}
}Method 3: Using JavaScript new keyword
In below code sample object has been created using Object constructor function.
var employee = new Object(); // Created employee object using new keywords and Object()
employee.name = 'Nishant';
employee.getName = function(){
return this.name;
}Note: As a best practices object literal way is used to create object over this method.
Question 29. Write a function called deepClone which takes an object and creates a object copy of it.
var newObject = deepClone(obj);Solution:
function deepClone(object){
var newObject = {};
for(var key in object){
if(typeof object[key] === 'object'){
newObject[key] = deepClone(object[key]);
}else{
newObject[key] = object[key];
}
}
return newObject;
}Explanation: We have been asked to do deep copy of object so What's basically it's mean ??. Let's understand in this way you have been given an object personalDetail this object contains some property which again a type of object here as you can see address is an object and phoneNumber in side an address is also an object. In simple term personalDetail is nested object(object inside object). So Here deep copy means we have to copy all the property of personalDetail object including nested object.
var personalDetail = {
name : 'Nishant',
address : {
location: 'xyz',
zip : '123456',
phoneNumber : {
homePhone: 8797912345,
workPhone : 1234509876
}
}
}So when we do deep clone then we should copy every property (including the nested object).
Suppose we have given an object
person
var person = {
name: 'Nishant',
age : 24
}Here the person object has a name and age property. Now we are trying to access the salary property which we haven't declared on the person object so while accessing it will return undefined. So how we will ensure whether property is undefined or not before performing some operation over it?
Explanation:
We can use typeof operator to check undefined
if(typeof someProperty === 'undefined'){
console.log('something is undefined here');
}Now we are trying to access salary property of person object.
if(typeof person.salary === 'undefined'){
console.log("salary is undefined here because we haven't declared");
}Question 31. Write a function called Clone which takes an object and creates a object copy of it but not copy deep property of object.
var objectLit = {foo : 'Bar'};
var cloneObj = Clone(obj); // Clone is the function which you have to write
console.log(cloneObj === Clone(objectLit)); // this should return false
console.log(cloneObj == Clone(objectLit)); // this should return truesolution:
function Clone(object){
var newObject = {};
for(var key in object){
newObject[key] = object[key];
}
return newObject;
}Suppose we have given an object
person
var person = {
name: 'Nishant',
age : 24
}here person object has name and age property. Now we are trying to access salary property which we haven't declared on person object so while accessing it will return undefined. So how we will ensure whether property is undefined or not before performing some operation over it.
Explanation:
We can use typeof operator to check undefined
if(typeof someProperty === 'undefined'){
console.log('something is undefined here');
}Now we are trying to access salary property of person object.
if(typeof person.salary === 'undefined'){
console.log("salary is undefined here because we haven't declared");
}Let say we have
personobject with property name and age
var person = {
name: 'Nishant',
age: 24
}Now we want to check whether name property exist in person object or not ?
In JavaScript object can have own property, in above example name and age is own property of person object. Object also have some of inherited property of base object like toString is inherited property of person object.
So how we will check whether property is own property or inherited property.
Method 1: We can use in operator on objet to check own property or inherited property.
console.log('name' in person); // checking own property print true
console.log('salary' in person); // checking undefined property print falsein operator also look into inherited property if it doesn't find property defined as own property. For instance If I check existence of toString property as we know that we haven't declared this property on person object so in operator look into there base property.
Here
console.log('toString' in person); // Will print trueIf we want to test property of object instance not inherited properties then we will use hasOwnProperty method of object instance.
console.log(person.hasOwnProperty('toString')); // print false
console.log(person.hasOwnProperty('name')); // print true
console.log(person.hasOwnProperty('salary')); // print falseWe always encounter in such situation where we need to know whether value is type of array or not.
For Instance : Below code perform some operation based value type
function(value){
if("value is an array"){
// Then perform some operation
}else{
// otherwise
}
}Let's discuss some way to detect an array in JavaScript.
Method 1 :
Duck typing test for array type detection
// Duck typing arrays
function isArray(value){
return typeof value.sort === 'function';
}As we can see above isArray method will return true if value object have sort method of type function. Now assume you have created a object with sort method
var bar = {
sort: function(){
// Some code
}
}Now when you check isArray(bar) then it will return true because bar object has sort method, But the fact is bar is not an array.
So method 1 is not a best way to detect an array as you can see it's not handle the case when some object has sort method.
Method 2:
Juriy Zaytsev (Also known as kangax) proposed an elegant solution to this.
function isArray(value){
return Object.prototype.toString.call(value) === '[object Array]';
}This approach is most popular way to detecting a value of type array in JavaScript and recommended to use. This approach relies on the fact that, native toString() method on a given value produce a standard string in all browser.
ECMAScript 5 has introduced Array.isArray() method to detect an array type value. The sole purpose of this method is accurately detecting whether a value is an array or not.
In many JavaScript libraries you may see below code for detecting an value of type array.
function(value){
// ECMAScript 5 feature
if(typeof Array.isArray === 'function'){
return Array.isArray(value);
}else{
return Object.prototype.toString.call(value) === '[object Array]';
}
}In Javascript Object are called as reference type, Any value other then primitive is definitely a reference type. There are several built-in reference type such as Object, Array, Function, Date, null and Error.
Detecting object using typeof operator
console.log(typeof {}); // object
console.log(typeof []); // object
console.log(typeof new Array()); // object
console.log(typeof null); // object
console.log(typeof new RegExp()); // object
console.log(typeof new Date()); // objectBut the downside of using typeof operator to detect an object is that typeof returns object for null (However this is fact that null is an object in JavaScript).
The best way to detect an object of specific reference type using instanceof operator.
Syntax : value instanceof constructor
//Detecting an array
if(value instanceof Array){
console.log("value is type of array");
}// Employee constructor function
function Employee(name){
this.name = name; // Public property
}
var emp1 = new Employee('John');
console.log(emp1 instanceof Employee); // trueinstanceof not only check the constructor which is used to create an object but also check it's prototype chain see below example.
console.log(emp1 instanceof Object); // trueObject-based inheritance also called prototypal inheritance in which we one object inherit from another object without invoking a constructor function.
The ECMAScript 5 Object.create() method is the easiest way for one object to inherit from another.
For Instance:
var employee = {
name: 'Nishant',
displayName: function () {
console.log(this.name);
}
};
var emp1 = Object.create(employee);
console.log(emp1.displayName()); // output "Nishant"Above example create a new object emp1 that inherits from employee. Here the inheritance occur as emp1's prototype is set to employee. After this emp1 is able to access the same properties and method on employee until new properties or method with the same name are defined.
For Instance: Defining displayName() method on emp1 automatically override the employee displayName.
emp1.displayName = function() {
console.log('xyz-Anonymous');
};
employee.displayName(); //Nishant
emp1.displayName();//xyz-AnonymousIn addition to this Object.create() method also allows to specify a second argument which is an object containing additional properties and methods to add to the new object.
For Example
var emp1 = Object.create(employee, {
name: {
value: "John"
}
});
emp1.displayName(); // "John"
employee.displayName(); // "Nishant"In above example, emp1 is created with it's own value for name, so calling displayName() method display "John" instead of "Nishant".
Object created in this manner give you full control over newly created object. You are free to add, remove any properties and method you want.
Type-based inheritance works with constructor function instead of object, It means we need to call constructor function of the object from which you want to inherit.
Let say we have Person class which has name, age, salary properties and incrementSalary() method.
function Person(name, age, salary) {
this.name = name;
this.age = age;
this.salary = salary;
this.incrementSalary = function (byValue) {
this.salary = this.salary + byValue;
};
}Now we wish to create Employee class which contains all the properties of Person class and wanted to add some additional properties into Employee class.
function Employee(company){
this.company = company;
}
//Prototypal Inheritance
Employee.prototype = new Person("Nishant", 24,5000);In above example, Employee type inherits from Person, which is called super types. It does so by assigning a new instance of Person to Employee prototype. After that, every instance of Employee inherits it's properties and methods from Person.
//Prototypal Inheritance
Employee.prototype = new Person("Nishant", 24,5000);
var emp1 = new Employee("Google");
console.log(emp1 instanceof Person); // true
console.log(emp1 instanceof Employee); // trueLet's understand Constructor inheritance
//Defined Person class
function Person(name){
this.name = name || "Nishant";
}
var obj = {};
// obj inherit Person class properties and method
Person.call(obj); // constructor inheritance
console.log(obj); // Object {name: "Nishant"}Here we saw calling Person.call(obj) define the name properties from Person to obj.
console.log(name in obj); // trueType-based inheritance is best used with developer defined constructor function rather than natively in JavaScript. In addition to this also allows flexibility in how we create similar type of object.
ECMAScript 5 introduce several methods to prevent modification of object which lock down object to ensure that no one, accidentally or otherwise, change functionality of Object.
There are three levels of preventing modification:
1: Prevent extensions :
No new properties or methods can be added to the object, but one can change the existing properties and method.
For Example:
var employee = {
name: "Nishant"
};
// lock the object
Object.preventExtensions(employee);
// Now try to change the employee object property name
employee.name = "John"; // work fine
//Now try to add some new property to the object
employee.age = 24; // fails silently unless it's inside the strict mode2: Seal :
It is same as prevent extension, in addition to this also prevent existing properties and methods from being deleted.
To seal an object, we use Object.seal() method. you can check whether an object is sealed or not using Object.isSealed();
var employee = {
name: "Nishant"
};
// Seal the object
Object.seal(employee);
console.log(Object.isExtensible(employee)); // false
console.log(Object.isSealed(employee)); // true
delete employee.name // fails silently unless it's in strict mode
// Trying to add new property will give an error
employee.age = 30; // fails silently unless in strict modewhen an object is sealed, its existing properties and methods can't be removed. Sealed object are also non-extensible.
3: Freeze :
Same as seal, In addition to this prevent existing properties methods from being modified (All properties and methods are read only).
To freeze an object, use Object.freeze() method. We can also determine whether an object is frozen using Object.isFrozen();
var employee = {
name: "Nishant"
};
//Freeze the object
Object.freeze(employee);
// Seal the object
Object.seal(employee);
console.log(Object.isExtensible(employee)); // false
console.log(Object.isSealed(employee)); // true
console.log(Object.isFrozen(employee)); // true
employee.name = "xyz"; // fails silently unless in strict mode
employee.age = 30; // fails silently unless in strict mode
delete employee.name // fails silently unless it's in strict modeFrozen objects are considered both non-extensible and sealed.
Recommended:
If you are decided to prevent modification, sealed, freeze the object then use in strict mode so that you can catch the error.
For Example:
"use strict";
var employee = {
name: "Nishant"
};
//Freeze the object
Object.freeze(employee);
// Seal the object
Object.seal(employee);
console.log(Object.isExtensible(employee)); // false
console.log(Object.isSealed(employee)); // true
console.log(Object.isFrozen(employee)); // true
employee.name = "xyz"; // fails silently unless in strict mode
employee.age = 30; // fails silently unless in strict mode
delete employee.name; // fails silently unless it's in strict modeQuestion 44. Write a log function which will add prefix (your message) to every message you log using console.log ?
For example, If you log
console.log("Some message")then output should be (your message) Some message
Logging error message or some informative message is always required when you dealing with client side JavaScript using console.log method. Some time you want to add some prefix to identify message generated log from your application hence you would like to prefix your app name in every console.log.
A general way to do this keep adding your app name in every console.log message like
console.log('your app name' + 'some error message');But doing in this way you have to write your app name everytime when you log message using console.
There are some best way we can achieve this
function appLog() {
var args = Array.prototype.slice.call(arguments);
args.unshift('your app name');
console.log.apply(console, args);
}
console.log(appLog("Some error message"));
//output of above console: 'your app name Some error message'For example: We can create string using string literal and using String constructor function.
// using string literal
var ltrlStr = "Hi I am string literal";
// using String constructor function
var objStr = new String("Hi I am string object");We can use typeof operator to test string literal and instanceof operator to test String object.
function isString(str) {
return typeof(str) == 'string' || str instanceof String;
}
var ltrlStr = "Hi I am string literal";
var objStr = new String("Hi I am string object");
console.log(isString(ltrlStr)); // true
console.log(isString(objStr)); // trueAnonymous functions basically used in following scenario.
-
No name is needed if function is only used in one place, then there is no need to add a name to function.
Let's take the example of setTimeout function
setTimeout(function(){ alert("Hello"); },1000);
Here there is no need of using named function when we are sure that function which will alert
hellowould use only once in application. -
Anonymous functions are declared inline and inline functions have advantages in the case that they can access variable in the parent scopes.
Let's take a example of event handler. Notify event of particular type (such as click) for a given object.
Let say we have HTML element (button) on which we want to add click event and when user do click on button we would like to execute some logic.
<button id="myBtn"></button>
Add Event Listener
var btn = document.getElementById('myBtn'); btn.addEventListener('click', function () { alert('button clicked'); });
Above example shows used of anonymous function as a callback function in event handler.
-
Passing anonymous function as a parameter to calling function.
Example:
// Function which will execute callback function function processCallback(callback){ if(typeof callback === 'function'){ callback(); } } // Call function and pass anonymous function as callback processCallback(function(){ alert("Hi I am anonymous callback function"); });
The best way to take decision for using anonymous function is to asked.
Will the function which I am going to define will use anywhere else.
If your answer is yes then go and create named function rather anonymous function.
Advantage of using anonymous function:
- It can reduce a bit of code, particularly in recursive function and in callback function.
- Avoid needless global namespace pollutions.
If you are coming from python/c# you might be using default value for function parameter incase value(formal parameter) has not been passed. For Instance :
// Define sentEmail function
// configuration : Configuration object
// provider : Email Service provider, Default would be gmail
def sentEmail(configuration, provider = 'Gmail'):
# Your code logicIn Pre ES6/ES2015
There are a lot of ways by which you can achieve this in pre ES2015.
Let's understand below code by which we achieved setting default parameter value.
Method 1: Setting default parameter value
function sentEmail(configuration, provider) {
// Set default value if user has not passed value for provider
provider = typeof provider !== 'undefined' ? provider : 'Gmail'
// Your code logic
;
}
// In this call we are not passing provider parameter value
sentEmail({
from: 'xyz@gmail.com',
subject: 'Test Email'
});
// Here we are passing Yahoo Mail as a provider value
sentEmail({
from: 'xyz@gmail.com',
subject: 'Test Email'
}, 'Yahoo Mail');Method 2: Setting default parameter value
function sentEmail(configuration, provider) {
// Set default value if user has not passed value for provider
provider = provider || 'Gmail'
// Your code logic
;
}
// In this call we are not passing provider parameter value
sentEmail({
from: 'xyz@gmail.com',
subject: 'Test Email'
});
// Here we are passing Yahoo Mail as a provider value
sentEmail({
from: 'xyz@gmail.com',
subject: 'Test Email'
}, 'Yahoo Mail');Let say you have two object
var person = {
name : 'John',
age : 24
}
var location = {
addressLine1 : 'Some Location x',
addressLine2 : 'Some Location y',
city : 'NewYork'
} Write merge function which will take two object and add all the own property of second object into first object.
merge(person , location);
/* Now person should have 5 properties
name , age , addressLine1 , addressLine2 , city */Method 1: Using ES6, Object assign method
function merge(toObj,fromObj){
return Object.assign(person,location);
}Method 2: Without using in-built function
function merge(toObj, fromObj) {
// Make sure both of the parameter is an object
if (typeof toObj === 'object' && typeof fromObj === 'object') {
for (var pro in fromObj) {
// Assign only own properties not inherited properties
if (fromObj.hasOwnProperty(pro)) {
// Assign property and value
toObj[pro] = fromObj[pro];
}
}
}else{
throw "Merge function can apply only on object";
}
}Object can have properties that don't show up when you iterate through object using for...in loop or using Object.keys() to get an array of property names. This properties is know as non-enumerable properties.
Let say we have following object
var person = {
name: 'John'
};
person.salary = '10000$';
person['country'] = 'USA';
console.log(Object.keys(person)); // ['name', 'salary', 'country']As we know that person object properties name, salary ,country are enumerable hence it's shown up when we called Object.keys(person).
To create a non-enumerable property we have to use Object.defineProperty(). This is a special method for creating non-enumerable property in JavaScript.
var person = {
name: 'John'
};
person.salary = '10000$';
person['country'] = 'USA';
// Create non-enumerable property
Object.defineProperty(person, 'phoneNo',{
value : '8888888888',
enumerable: false
})
Object.keys(person); // ['name', 'salary', 'country']In above example phoneNo property didn't show up because we made it non-enumerable by setting enumerable:false
Now let's try to change value of phoneNo
person.phoneNo = '7777777777'; Changing non-enumerable property value will return error in strict mode. In non-strict mode it won't through any error but it won't change the value of phoneNo.
Bonus
Object.defineProperty() is also let you create read-only properties as we saw above, we are not able to modify phoneNo value of a person object.
Function binding falls in advance JavaScript category and this is very popular technique to use in conjunction with event handler and callback function to preserve code execution context while passing function as a parameter.
Let's consider the following example:
var clickHandler = {
message: 'click event handler',
handleClick: function(event) {
console.log(this.message);
}
};
var btn = document.getElementById('myBtn');
// Add click event to btn
btn.addEventListener('click', clickHandler.handleClick);Here in this example clickHandler object is created which contain message properties and handleClick method.
We have assigned handleClick method to a DOM button, which will be executed in response of click. When the button is clicked, then handleClick method is being called and console message. Here console.log should log the click event handler message but it actually log undefined.
The problem of displaying undefined is because of the execution context of clickHandler.handleClick method is not being saved hence this pointing to button btn object. We can fix this issue using bind method.
var clickHandler = {
message: 'click event handler',
handleClick: function(event) {
console.log(this.message);
}
};
var btn = document.getElementById('myBtn');
// Add click event to btn and bind the clickHandler object
btn.addEventListener('click', clickHandler.handleClick.bind(clickHandler));bind method is available to all the function similar to call and apply method which take argument value of this.
- Some Value
- Undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 4) ReferenceError: employeeId is not defined
console.log(employeeId);
var employeeId = '19000';- Some Value
- undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 2) undefined
var employeeId = '1234abe';
(function(){
console.log(employeeId);
var employeeId = '122345';
})();- '122345'
- undefined
- Type Error
- ReferenceError: employeeId is not defined
Answer: 2) undefined
var employeeId = '1234abe';
(function() {
console.log(employeeId);
var employeeId = '122345';
(function() {
var employeeId = 'abc1234';
}());
}());- '122345'
- undefined
- '1234abe'
- ReferenceError: employeeId is not defined
Answer: 2) undefined
(function() {
console.log(typeof displayFunc);
var displayFunc = function(){
console.log("Hi I am inside displayFunc");
}
}());- undefined
- function
- 'Hi I am inside displayFunc'
- ReferenceError: displayFunc is not defined
Answer: 1) undefined
var employeeId = 'abc123';
function foo(){
employeeId = '123bcd';
return;
}
foo();
console.log(employeeId);- undefined
- '123bcd'
- 'abc123'
- ReferenceError: employeeId is not defined
Answer: 2) '123bcd'
var employeeId = 'abc123';
function foo() {
employeeId = '123bcd';
return;
function employeeId() {}
}
foo();
console.log(employeeId);- undefined
- '123bcd'
- 'abc123'
- ReferenceError: employeeId is not defined
Answer: 3) 'abc123'
var employeeId = 'abc123';
function foo() {
employeeId();
return;
function employeeId() {
console.log(typeof employeeId);
}
}
foo();- undefined
- function
- string
- ReferenceError: employeeId is not defined
Answer: 2) 'function'
function foo() {
employeeId();
var product = 'Car';
return;
function employeeId() {
console.log(product);
}
}
foo();- undefined
- Type Error
- 'Car'
- ReferenceError: product is not defined
Answer: 1) undefined
(function foo() {
bar();
function bar() {
abc();
console.log(typeof abc);
}
function abc() {
console.log(typeof bar);
}
}());- undefined undefined
- Type Error
- function function
- ReferenceError: bar is not defined
Answer: 3) function function
(function() {
'use strict';
var person = {
name: 'John'
};
person.salary = '10000$';
person['country'] = 'USA';
Object.defineProperty(person, 'phoneNo', {
value: '8888888888',
enumerable: true
})
console.log(Object.keys(person));
})();- Type Error
- undefined
- ["name", "salary", "country", "phoneNo"]
- ["name", "salary", "country"]
Answer: 3) ["name", "salary", "country", "phoneNo"]
(function() {
'use strict';
var person = {
name: 'John'
};
person.salary = '10000$';
person['country'] = 'USA';
Object.defineProperty(person, 'phoneNo', {
value: '8888888888',
enumerable: false
})
console.log(Object.keys(person));
})();- Type Error
- undefined
- ["name", "salary", "country", "phoneNo"]
- ["name", "salary", "country"]
Answer: 4) ["name", "salary", "country"]
(function() {
var objA = {
foo: 'foo',
bar: 'bar'
};
var objB = {
foo: 'foo',
bar: 'bar'
};
console.log(objA == objB);
console.log(objA === objB);
}());- false true
- false false
- true false
- true true
Answer: 2) false false
(function() {
var objA = new Object({foo: "foo"});
var objB = new Object({foo: "foo"});
console.log(objA == objB);
console.log(objA === objB);
}());- false true
- false false
- true false
- true true
Answer: 2) false false
(function() {
var objA = Object.create({
foo: 'foo'
});
var objB = Object.create({
foo: 'foo'
});
console.log(objA == objB);
console.log(objA === objB);
}());- false true
- false false
- true false
- true true
Answer: 2) false false
(function() {
var objA = Object.create({
foo: 'foo'
});
var objB = Object.create(objA);
console.log(objA == objB);
console.log(objA === objB);
}());- false true
- false false
- true false
- true true
Answer: 2) false false
(function() {
var objA = Object.create({
foo: 'foo'
});
var objB = Object.create(objA);
console.log(objA.toString() == objB.toString());
console.log(objA.toString() === objB.toString());
}());- false true
- false false
- true false
- true true
Answer: 4) true true
(function() {
var objA = Object.create({
foo: 'foo'
});
var objB = objA;
console.log(objA == objB);
console.log(objA === objB);
console.log(objA.toString() == objB.toString());
console.log(objA.toString() === objB.toString());
}());- true true true false
- true false true true
- true true true true
- true true false false
Answer: 3) true true true true
(function() {
var objA = Object.create({
foo: 'foo'
});
var objB = objA;
objB.foo = 'bar';
console.log(objA.foo);
console.log(objB.foo);
}());- foo bar
- bar bar
- foo foo
- bar foo
Answer: 2) bar bar
(function() {
var objA = Object.create({
foo: 'foo'
});
var objB = objA;
objB.foo = 'bar';
delete objA.foo;
console.log(objA.foo);
console.log(objB.foo);
}());- foo bar
- bar bar
- foo foo
- bar foo
Answer: 3) foo foo
(function() {
var objA = {
foo: 'foo'
};
var objB = objA;
objB.foo = 'bar';
delete objA.foo;
console.log(objA.foo);
console.log(objB.foo);
}());- foo bar
- undefined undefined
- foo foo
- undefined bar
Answer: 2) undefined undefined
(function() {
var array = new Array('100');
console.log(array);
console.log(array.length);
}());- undefined undefined
- [undefined × 100] 100
- ["100"] 1
- ReferenceError: array is not defined
Answer: 3) ["100"] 1
(function() {
var array1 = [];
var array2 = new Array(100);
var array3 = new Array(['1',2,'3',4,5.6]);
console.log(array1);
console.log(array2);
console.log(array3);
console.log(array3.length);
}());- [] [] [Array[5]] 1
- [] [undefined × 100] Array[5] 5
- [] [] ['1',2,'3',4,5.6] 5
- [] [] [Array[5]] 5
Answer: 1) [] [] [Array[5]] 1
(function () {
var array = new Array('a', 'b', 'c', 'd', 'e');
array[10] = 'f';
delete array[10];
console.log(array.length);
}());- 11
- 5
- 6
- undefined
Answer: 1) 11
(function(){
var animal = ['cow','horse'];
animal.push('cat');
animal.push('dog','rat','goat');
console.log(animal.length);
})();- 4
- 5
- 6
- undefined
Answer: 3) 6
(function(){
var animal = ['cow','horse'];
animal.push('cat');
animal.unshift('dog','rat','goat');
console.log(animal);
})();- 10000 [ 'dog', 'rat', 'goat', 'cow', 'horse', 'cat' ]
- [ 'cow', 'horse', 'cat', 'dog', 'rat', 'goat' ]
- Type Error
- undefined
Answer: 1) [ 'dog', 'rat', 'goat', 'cow', 'horse', 'cat' ]
(function(){
var array = [1,2,3,4,5];
console.log(array.indexOf(2));
console.log([{name: 'John'},{name : 'John'}].indexOf({name:'John'}));
console.log([[1],[2],[3],[4]].indexOf([3]));
console.log("abcdefgh".indexOf('e'));
})();- 1 -1 -1 4
- 1 0 -1 4
- 1 -1 -1 -1
- 1 undefined -1 4
Answer: 1) 1 -1 -1 4
(function(){
var array = [1,2,3,4,5,1,2,3,4,5,6];
console.log(array.indexOf(2));
console.log(array.indexOf(2,3));
console.log(array.indexOf(2,10));
})();- 1 -1 -1
- 1 6 -1
- 1 1 -1
- 1 undefined undefined
Answer: 2) 1 6 -1
(function(){
var numbers = [2,3,4,8,9,11,13,12,16];
var even = numbers.filter(function(element, index){
return element % 2 === 0;
});
console.log(even);
var containsDivisibleby3 = numbers.some(function(element, index){
return element % 3 === 0;
});
console.log(containsDivisibleby3);
})();- [ 2, 4, 8, 12, 16 ] [ 0, 3, 0, 0, 9, 0, 12]
- [ 2, 4, 8, 12, 16 ] [ 3, 9, 12]
- [ 2, 4, 8, 12, 16 ] true
- [ 2, 4, 8, 12, 16 ] false
Answer: 3) [ 2, 4, 8, 12, 16 ] true
(function(){
var containers = [2,0,false,"", '12', true];
var containers = containers.filter(Boolean);
console.log(containers);
var containers = containers.filter(Number);
console.log(containers);
var containers = containers.filter(String);
console.log(containers);
var containers = containers.filter(Object);
console.log(containers);
})();- [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ]
- [false, true] [ 2 ] ['12'] [ ]
- [2,0,false,"", '12', true] [2,0,false,"", '12', true] [2,0,false,"", '12', true] [2,0,false,"", '12', true]
- [ 2, '12', true ] [ 2, '12', true, false ] [ 2, '12', true,false ] [ 2, '12', true,false]
Answer: 1) [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ] [ 2, '12', true ]
(function(){
var list = ['foo','bar','john','ritz'];
console.log(list.slice(1));
console.log(list.slice(1,3));
console.log(list.slice());
console.log(list.slice(2,2));
console.log(list);
})();- [ 'bar', 'john', 'ritz' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
- [ 'bar', 'john', 'ritz' ] [ 'bar', 'john','ritz ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
- [ 'john', 'ritz' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
- [ 'foo' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
Answer: 1) [ 'bar', 'john', 'ritz' ] [ 'bar', 'john' ] [ 'foo', 'bar', 'john', 'ritz' ] [] [ 'foo', 'bar', 'john', 'ritz' ]
(function(){
var list = ['foo','bar','john'];
console.log(list.splice(1));
console.log(list.splice(1,2));
console.log(list);
})();- [ 'bar', 'john' ] [] [ 'foo' ]
- [ 'bar', 'john' ] [] [ 'bar', 'john' ]
- [ 'bar', 'john' ] [ 'bar', 'john' ] [ 'bar', 'john' ]
- [ 'bar', 'john' ] [] []
Answer: 1. [ 'bar', 'john' ] [] [ 'foo' ]
(function(){
var arrayNumb = [2, 8, 15, 16, 23, 42];
arrayNumb.sort();
console.log(arrayNumb);
})();- [2, 8, 15, 16, 23, 42]
- [42, 23, 26, 15, 8, 2]
- [ 15, 16, 2, 23, 42, 8 ]
- [ 2, 8, 15, 16, 23, 42 ]
Answer: 3. [ 15, 16, 2, 23, 42, 8 ]
function funcA(){
console.log("funcA ", this);
(function innerFuncA1(){
console.log("innerFunc1", this);
(function innerFunA11(){
console.log("innerFunA11", this);
})();
})();
}
console.log(funcA());- funcA Window {...} innerFunc1 Window {...} innerFunA11 Window {...}
- undefined
- Type Error
- ReferenceError: this is not defined
Answer: 1)
var obj = {
message: "Hello",
innerMessage: !(function() {
console.log(this.message);
})()
};
console.log(obj.innerMessage);- ReferenceError: this.message is not defined
- undefined
- Type Error
- undefined true
Answer: 4) undefined true
var obj = {
message: "Hello",
innerMessage: function() {
console.log(this.message);
}
};
console.log(obj.innerMessage());- Hello
- undefined
- Type Error
- ReferenceError: this.message is not defined
Answer: 1) Hello
var obj = {
message: 'Hello',
innerMessage: function () {
(function () {
console.log(this.message);
}());
}
};
console.log(obj.innerMessage());- Type Error
- Hello
- undefined
- ReferenceError: this.message is not defined
Answer: 3) undefined
var obj = {
message: 'Hello',
innerMessage: function () {
var self = this;
(function () {
console.log(self.message);
}());
}
};
console.log(obj.innerMessage());- Type Error
- 'Hello'
- undefined
- ReferenceError: self.message is not defined
Answer: 2) 'Hello'
function myFunc(){
console.log(this.message);
}
myFunc.message = "Hi John";
console.log(myFunc());- Type Error
- 'Hi John'
- undefined
- ReferenceError: this.message is not defined
Answer: 3) undefined
function myFunc(){
console.log(myFunc.message);
}
myFunc.message = "Hi John";
console.log(myFunc());- Type Error
- 'Hi John'
- undefined
- ReferenceError: this.message is not defined
Answer: 2) 'Hi John'
function myFunc() {
myFunc.message = 'Hi John';
console.log(myFunc.message);
}
console.log(myFunc());- Type Error
- 'Hi John'
- undefined
- ReferenceError: this.message is not defined
Answer: 2) 'Hi John'
function myFunc(param1,param2) {
console.log(myFunc.length);
}
console.log(myFunc());
console.log(myFunc("a","b"));
console.log(myFunc("a","b","c","d"));- 2 2 2
- 0 2 4
- undefined
- ReferenceError
Answer: a) 2 2 2
function myFunc() {
console.log(arguments.length);
}
console.log(myFunc());
console.log(myFunc("a","b"));
console.log(myFunc("a","b","c","d"));- 2 2 2
- 0 2 4
- undefined
- ReferenceError
Answer: 2) 0 2 4
function Person(name, age){
this.name = name || "John";
this.age = age || 24;
this.displayName = function(){
console.log(this.name);
}
}
Person.name = "John";
Person.displayName = function(){
console.log(this.name);
}
var person1 = new Person('John');
person1.displayName();
Person.displayName();- John Person
- John John
- John undefined
- John John
Answer: 1) John Person
function passWordMngr() {
var password = '12345678';<
10000
/span>
this.userName = 'John';
return {
pwd: password
};
}
// Block End
var userInfo = passWordMngr();
console.log(userInfo.pwd);
console.log(userInfo.userName);- 12345678 Window
- 12345678 John
- 12345678 undefined
- undefined undefined
Answer: 3) 12345678 undefined
var employeeId = 'aq123';
function Employee() {
this.employeeId = 'bq1uy';
}
console.log(Employee.employeeId);- Reference Error
- aq123
- bq1uy
- undefined
Answer: 4) undefined
var employeeId = 'aq123';
function Employee() {
this.employeeId = 'bq1uy';
}
console.log(new Employee().employeeId);
Employee.prototype.employeeId = 'kj182';
Employee.prototype.JobId = '1BJKSJ';
console.log(new Employee().JobId);
console.log(new Employee().employeeId);- bq1uy 1BJKSJ bq1uy undefined
- bq1uy 1BJKSJ bq1uy
- bq1uy 1BJKSJ kj182
- undefined 1BJKSJ kj182
Answer: 2) bq1uy 1BJKSJ bq1uy
var employeeId = 'aq123';
(function Employee() {
try {
throw 'foo123';
} catch (employeeId) {
console.log(employeeId);
}
console.log(employeeId);
}());- foo123 aq123
- foo123 foo123
- aq123 aq123
- foo123 undefined
Answer: 1) foo123 aq123
(function() {
var greet = 'Hello World';
var toGreet = [].filter.call(greet, function(element, index) {
return index > 5;
});
console.log(toGreet);
}());- Hello World
- undefined
- World
- [ 'W', 'o', 'r', 'l', 'd' ]
Answer: 4) [ 'W', 'o', 'r', 'l', 'd' ]
(function() {
var fooAccount = {
name: 'John',
amount: 4000,
deductAmount: function(amount) {
this.amount -= amount;
return 'Total amount left in account: ' + this.amount;
}
};
var barAccount = {
name: 'John',
amount: 6000
};
var withdrawAmountBy = function(totalAmount) {
return fooAccount.deductAmount.bind(barAccount, totalAmount);
};
console.log(withdrawAmountBy(400)());
console.log(withdrawAmountBy(300)());
}());- Total amount left in account: 5600 Total amount left in account: 5300
- undefined undefined
- Total amount left in account: 3600 Total amount left in account: 3300
- Total amount left in account: 5600 Total amount left in account: 5600
Answer: 1) Total amount left in account: 5600 Total amount left in account: 5300
(function() {
var fooAccount = {
name: 'John',
amount: 4000,
deductAmount: function(amount) {
this.amount -= amount;
return this.amount;
}
};
var barAccount = {
name: 'John',
amount: 6000
};
var withdrawAmountBy = function(totalAmount) {
return fooAccount.deductAmount.apply(barAccount, [totalAmount]);
};
console.log(withdrawAmountBy(400));
console.log(withdrawAmountBy(300));
console.log(withdrawAmountBy(200));
}());- 5600 5300 5100
- 3600 3300 3100
- 5600 3300 5100
- undefined undefined undefined
Answer: 1) 5600 5300 5100
(function() {
var fooAccount = {
name: 'John',
amount: 6000,
deductAmount: function(amount) {
this.amount -= amount;
return this.amount;
}
};
var barAccount = {
name: 'John',
amount: 4000
};
var withdrawAmountBy = function(totalAmount) {
return fooAccount.deductAmount.call(barAccount, totalAmount);
};
console.log(withdrawAmountBy(400));
console.log(withdrawAmountBy(300));
console.log(withdrawAmountBy(200));
}());- 5600 5300 5100
- 3600 3300 3100
- 5600 3300 5100
- undefined undefined undefined
Answer: 2) 3600 3300 3100
(function greetNewCustomer() {
console.log('Hello ' + this.name);
}.bind({
name: 'John'
})());- Hello John
- Reference Error
- Window
- undefined
Answer: 1) Hello John
(function greetNewCustomer() {
console.log('Hello ' + this.name);
}.bind({
name: 'John'
})());- Hello John
- Reference Error
- Window
- undefined
Answer: 1) Hello John
function getDataFromServer(apiUrl){
var name = "John";
return {
then : function(fn){
fn(name);
}
}
}
getDataFromServer('www.google.com').then(function(name){
console.log(name);
});- John
- undefined
- Reference Error
- fn is not defined
Answer: 1) John
(function(){
var arrayNumb = [2, 8, 15, 16, 23, 42];
Array.prototype.sort = function(a,b){
return a - b;
};
arrayNumb.sort();
console.log(arrayNumb);
})();
(function(){
var numberArray = [2, 8, 15, 16, 23, 42];
numberArray.sort(function(a,b){
if(a == b){
return 0;
}else{
return a < b ? -1 : 1;
}
});
console.log(numberArray);
})();
(function(){
var numberArray = [2, 8, 15, 16, 23, 42];
numberArray.sort(function(a,b){
return a-b;
});
console.log(numberArray);
})();- [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]
- undefined undefined undefined
- [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2] [42, 23, 16, 15, 8, 2]
- Reference Error
Answer: 1) [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ] [ 2, 8, 15, 16, 23, 42 ]
(function(){
function sayHello(){
var name = "Hi John";
return
{
fullName: name
}
}
console.log(sayHello().fullName);
})();- Hi John
- undefined
- Reference Error
- Uncaught TypeError: Cannot read property 'fullName' of undefined
Answer: 4) Uncaught TypeError: Cannot read property 'fullName' of undefined
function getNumber(){
return (2,4,5);
}
var numb = getNumber();
console.log(numb);- 5
- undefined
- 2
- (2,4,5)
Answer: 1) 5
function getNumber(){
return;
}
var numb = getNumber();
console.log(numb);- null
- undefined
- ""
- 0
Answer: 2) undefined
function mul(x){
return function(y){
return [x*y, function(z){
return x*y + z;
}];
}
}
console.log(mul(2)(3)[0]);
console.log(mul(2)(3)[1](4));- 6, 10
- undefined undefined
- Reference Error
- 10, 6
Answer: 1) 6, 10
function mul(x) {
return function(y) {
return {
result: x * y,
sum: function(z) {
return x * y + z;
}
};
};
}
console.log(mul(2)(3).result);
console.log(mul(2)(3).sum(4));- 6, 10
- undefined undefined
- Reference Error
- 10, 6
Answer: 1) 6, 10
function mul(x) {
return function(y) {
return function(z) {
return function(w) {
return function(p) {
return x * y * z * w * p;
};
};
};
};
}
console.log(mul(2)(3)(4)(5)(6));- 720
- undefined
- Reference Error
- Type Error
Answer: 1) 720
We always appreciate your feedback on how the book can be improved, and more questions can be added. If you think you have some question then please add that and open a pull request.
This book is released under a Creative Commons Attribution-Noncommercial- No Derivative Works 3.0 United States License.
What this means it that the project is free to read and use, but the license does not permit commercial use of the material (i.e you can freely print out the questions for your own use, but you can't sell it). I'm trying to best to publish all of my books in a free + purchased (if you would like to support these projects) form so I would greatly appreciate it if you would respect these terms.
Copyright Iurii Katkov and Nishant Kumar, 2017.