About stdlib...

We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.

The library is fully decomposable, being architected in such a way that you can swap out and mix and match APIs and functionality to cater to your exact preferences and use cases.

When you use stdlib, you can be absolutely certain that you are using the most thorough, rigorous, well-written, studied, documented, tested, measured, and high-quality code out there.

To join us in bringing numerical computing to the web, get started by checking us out on GitHub, and please consider financially supporting stdlib. We greatly appreciate your continued support!

inmapAsync

Invoke a function for each element in a collection and update the collection in-place.

Installation

npm install @stdlib/utils-async-inmap

Alternatively,

• To load the package in a website via a script tag without installation and bundlers, use the ES Module available on the esm branch (see README).
• If you are using Deno, visit the deno branch (see README for usage intructions).
• For use in Observable, or in browser/node environments, use the Universal Module Definition (UMD) build available on the umd branch (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.

Usage

var inmapAsync = require( '@stdlib/utils-async-inmap' );

inmapAsync( collection, [options,] fcn, done )

Invokes a function for each element in a collection and updates the collection in-place.

function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    console.log( collection );
}

var arr = [ 3000, 2500, 1000 ];

inmapAsync( arr, fcn, done );
/*
    1000
    2500
    3000
    true
    [ 0, 2500, 2000 ]
*/

The next callback accepts two arguments: error and result. The second argument to the next callback is used to update the collection element for the corresponding collection index, thus mutating the input collection.

function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        next( null, 'beep: '+index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection );
    // => [ 'beep: 0', 'beep: 1', 'beep: 2' ]
}

var arr = [ 3000, 2500, 1000 ];

inmapAsync( arr, fcn, done );

If the next callback is called with an error argument, the input collection may be partially mutated.

function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        if ( index === 1 ) {
            return next( new Error( 'boop' ) );
        }
        next( null, 'beep: '+index );
    }
}

function done( error, collection ) {
    if ( error ) {
        // Ignore error...
    }
    console.log( collection );
    // => [ 'beep: 0', 2000, 3000 ]
}

var arr = [ 1000, 2000, 3000 ];

inmapAsync( arr, fcn, done );

The function accepts the following options:

• limit: the maximum number of pending invocations at any one time. Default: infinity.
• series: boolean indicating whether to sequentially invoke fcn for each collection element. If true, the function sets options.limit=1. Default: false.
• thisArg: the execution context for fcn.

By default, all elements are processed concurrently, which means that the function does not guarantee completion order. To process each collection element sequentially, set the series option to true.

function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    console.log( collection );
}

var arr = [ 3000, 2500, 1000 ];

var opts = {
    'series': true
};

inmapAsync( arr, opts, fcn, done );
/* =>
    3000
    2500
    1000
    true
    [ 0, 2500, 2000 ]
*/

To limit the maximum number of pending function invocations, set the limit option.

function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    console.log( collection );
}

var arr = [ 3000, 2500, 1000 ];

var opts = {
    'limit': 2
};

inmapAsync( arr, opts, fcn, done );
/* =>
    2500
    3000
    1000
    true,
    [ 0, 2500, 2000 ]
*/

To set the execution context of fcn, set the thisArg option.

function fcn( value, index, next ) {
    this.count += 1;
    setTimeout( onTimeout, value );
    function onTimeout() {
        next( null, value*index );
    }
}

var arr = [ 3000, 2500, 1000 ];

var context = {
    'count': 0
};

var opts = {
    'thisArg': context
};

inmapAsync( arr, opts, fcn, done );

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    // => true

    console.log( collection );
    // => [ 0, 2500, 2000 ]

    console.log( context.count );
    // => 3
}

When invoked, fcn is provided a maximum of four arguments:

• value: collection value.
• index: collection index.
• collection: the input collection.
• next: a callback which should be called once fcn has finished processing a collection value.

The actual number of provided arguments depends on function length. If fcn accepts two arguments, fcn is provided value and next. If fcn accepts three arguments, fcn is provided value, index, and next. For every other fcn signature, fcn is provided all four arguments.

function fcn( value, i, collection, next ) {
    console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*i );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection === arr );
    // => true

    console.log( collection );
    // => [ 0, 2500, 2000 ]
}

var arr = [ 3000, 2500, 1000 ];

inmapAsync( arr, fcn, done );
/* =>
    collection: 3000,2500,1000. 0: 3000
    collection: 3000,2500,1000. 1: 2500
    collection: 3000,2500,1000. 2: 1000
    1000
    2500
    3000
*/

inmapAsync.factory( [options,] fcn )

Returns a function which invokes a function once for each element in a collection.

function fcn( value, index, next ) {
    setTimeout( onTimeout, value );
    function onTimeout() {
        console.log( value );
        next( null, value*index );
    }
}

function done( error, collection ) {
    if ( error ) {
        throw error;
    }
    console.log( collection );
}

var f = inmapAsync.factory( fcn );

var arr1 = [ 3000, 2500, 1000 ];

f( arr1, done );
/* =>
    1000
    2500
    3000
    [ 0, 2500, 2000 ]
*/

var arr2 = [ 300, 250, 100 ];

f( arr2, done );
/* =>
    100
    250
    300
    [ 0, 250, 200 ]
*/

The function accepts the same options as inmapAsync().

Notes

• A collection may be either an Array, Typed Array, or an array-like Object (excluding strings and functions).
• If a provided function calls the next callback with a truthy error argument, the function suspends execution and immediately calls the done callback for subsequent error handling.
• The function invokes the done callback with the input collection provided as the second argument.
• The function modifies collection elements in-place.
• The function does not support dynamic collection resizing.
• The function does not skip undefined elements.
• Neither inmapAsync nor the function returned by the factory method guarantee asynchronous execution. To guarantee asynchrony, wrap the done callback 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).

Examples

var resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs-read-file' );
var inmapAsync = require( '@stdlib/utils-async-inmap' );

var files = [
    resolve( __dirname, 'package.json' ),
    resolve( __dirname, 'README.md' )
];

function done( error, results ) {
    if ( error ) {
        throw error;
    }
    console.log( results );
}

function read( file, next ) {
    var opts = {
        'encoding': 'utf8'
    };
    readFile( file, opts, onFile );

    function onFile( error, data ) {
        if ( error ) {
            error = new Error( 'unable to read file: '+file );
            return next( error );
        }
        next( null, data );
    }
}

inmapAsync( files, read, done );

---

See Also

• @stdlib/utils-async/for-each: invoke a function once for each element in a collection.
• @stdlib/utils-async/inmap-right: invoke a function for each element in a collection and update the collection in-place, iterating from right to left.
• @stdlib/utils-inmap: invoke a function for each element in a collection and update the collection in-place.

---

Notice

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.

Community

---

License

See LICENSE.

Copyright

Copyright © 2016-2025. The Stdlib Authors.