Execution Context

Everything happens inside a Execution context

The browser's javascript engine creates a special environment to handle the transformation and execution of javascript code.

Execution context has a two component

Memory component - is also knows as variable environment which contains variables and functions as a key-value pairs.
Code component - is also known as a thread of execution where javascript code is executed one line at a time

Javascript is synchronous single threaded language

single threaded means that javascript can execute one command at a time
Synchronous single-threaded means that it can execute one command in order at a time.

When JavaScript code runs Global Execution Context (GEC) is created. Execution of code is happening in two phases:

Phase 1 (Creation Phase) is allocating memory to the variables and functions. For all variables value stored in memory is undefined and functions are copied in memory.
In Phase 2 (Execution Phase) JavaScript code is executed line by line and it is defining variables in memory here. Every function invoked creates its own local execution context, which is put in call stack (explained in the next chapter).

var n = 2;
function square(num) {
    var ans = num * num;
    return ans;
}
var x = square(n);
var y = square(4);

when you invoke a function, a whole new execution context is created