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Map keys from one object to a new object having the same values.
npm install @stdlib/utils-async-map-keysAlternatively,
- To load the package in a website via a
scripttag without installation and bundlers, use the ES Module available on theesmbranch (see README). - If you are using Deno, visit the
denobranch (see README for usage intructions). - For use in Observable, or in browser/node environments, use the Universal Module Definition (UMD) build available on the
umdbranch (see README).
The branches.md file summarizes the available branches and displays a diagram illustrating their relationships.
To view installation and usage instructions specific to each branch build, be sure to explicitly navigate to the respective README files on each branch, as linked to above.
var mapKeysAsync = require( '@stdlib/utils-async-map-keys' );Map keys from one object to a new object having the same values.
function transform( key, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, key+':beep' );
}
}
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2 }
}
var obj = {
'a': 1,
'b': 2
};
mapKeysAsync( obj, transform, done );The next callback accepts two arguments: error and key. The second argument to the next callback is the transformed property name. If a transform function calls the next callback with a truthy error argument, the function stops processing any additional own properties and calls the done callback for error processing.
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
if ( value === 1 ) {
return next( new Error( 'boop' ) );
}
next( null, key+':beep' );
}
}
function done( error ) {
if ( error ) {
console.error( error.message );
// => 'boop'
}
}
var obj = {
'a': 1,
'b': 2
};
mapKeysAsync( obj, transform, done );The function accepts the following options:
limit: the maximum number of pending invocations at any one time. Default:infinity.series:booleanindicating whether to sequentially invoke thetransformfunction for each own property. Iftrue, the function setsoptions.limit=1. Default:false.thisArg: the execution context forfcn.
By default, all properties are processed concurrently, which means that the function does not guarantee completion order. To process each property sequentially, set the series option to true.
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2 }
}
var obj = {
'a': 1,
'b': 2
};
var opts = {
'series': true
};
mapKeysAsync( obj, opts, transform, done );To limit the maximum number of pending function invocations, set the limit option.
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2, 'c:beep': 3 }
}
var obj = {
'a': 1,
'b': 2,
'c': 3
};
var opts = {
'limit': 2
};
mapKeysAsync( obj, opts, transform, done );To set the execution context of the transform function, set the thisArg option.
function transform( key, value, next ) {
this.count += 1;
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}
var obj = {
'a': 1,
'b': 2,
'c': 3
};
var context = {
'count': 0
};
var opts = {
'thisArg': context
};
mapKeysAsync( obj, opts, transform, done );
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2, 'c:beep': 3 }
console.log( context.count );
// => 3
}When invoked, the transform f
5CC0
unction is provided a maximum of four arguments:
key: object key.value: object value corresponding tokey.obj: source object.next: a callback which should be called once thetransformfunction has finished processing a property.
The actual number of provided arguments depends on function length. If the transform function accepts two arguments, the transform function is provided key and next. If the transform function accepts three arguments, the transform function is provided key, value, and next. For every other transform function signature, the transform function is provided all four arguments.
function transform( key, value, obj, next ) {
console.log( 'obj: %s. %s: %d', JSON.stringify( obj ), key, value );
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':'+value );
}
}
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
}
var obj = {
'a': 1,
'b': 2
};
mapKeysAsync( obj, transform, done );
/* => e.g.,
obj: {"a": 1, "b": 2}. a: 1
obj: {"a": 1, "b": 2}. b: 2
{ 'a:1': 1, 'b:2': 2 }
*/Returns a function which invokes a transform function once for each own property.
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
}
var f = mapKeysAsync.factory( transform );
var obj1 = {
'a': 1,
'b': 2
};
f( obj1, done );
// => { 'a:beep': 1, 'b:beep': 2 }
var obj2 = {
'c': 3,
'd': 4
};
f( obj2, done );
// => { 'c:beep': 3, 'd:beep': 4 }The function accepts the same options as mapKeysAsync().
- If a provided function calls the
nextcallback with a truthyerrorargument, the function suspends execution and immediately calls thedonecallback for subsequenterrorhandling. - If provided an empty
object, the function calls thedonecallback with an emptyobject. - Key iteration order is not guaranteed, as
objectkey enumeration is not specified according to the ECMAScript specification. In practice, however, most engines use insertion order to sort anobject's keys, thus allowing for iteration order. - Key insertion order is not guaranteed.
- The value returned by a
transformfunction should be a value which can be serialized as anobjectkey. - The function only maps own properties. Hence, the function does not map inherited properties.
- The function shallow copies key values.
- Neither
mapKeysAsyncnor the function returned by thefactorymethod guarantee asynchronous execution. To guarantee asynchrony, wrap thedonecallback in a function which either executes at the end of the current stack (e.g.,nextTick) or during a subsequent turn of the event loop (e.g.,setImmediate,setTimeout).
var resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs-read-file' );
var mapKeysAsync = require( '@stdlib/utils-async-map-keys' );
var files = {
'file1': resolve( __dirname, 'package.json' ),
'file2': resolve( __dirname, 'README.md' )
};
function read( key, value, next ) {
var opts = {
'encoding': 'utf8'
};
readFile( value, opts, onFile );
function onFile( error ) {
if ( error ) {
return next( error, key+':unreadable' );
}
next( null, key+':readable' );
}
}
function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
}
mapKeysAsync( files, read, done );@stdlib/utils-map-keys: map keys from one object to a new object having the same values.@stdlib/utils-async/map-values: map values from one object to a new object having the same keys.
This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.
For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.
See LICENSE.
Copyright © 2016-2025. The Stdlib Authors.
