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forEachRight

Invoke a function for each element in a collection, iterating from the right to left.

Installation

npm install @stdlib/utils-for-each-right

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 forEachRight = require( '@stdlib/utils-for-each-right' );

forEachRight( collection, fcn[, thisArg ] )

Invokes a function for each element in a collection, iterating from right to left.

function log( value, index ) {
    console.log( '%s: %d', index, value );
}

var arr = [ 1, 2, 3, 4 ];

forEachRight( arr, log );
/* =>
    3: 4
    2: 3
    1: 2
    0: 1
*/

The invoked function is provided three arguments:

• value: collection element.
• index: collection index.
• collection: input collection.

Basic support for dynamic collections is provided. Note, however, that index incrementation is not guaranteed to be monotonically decreasing.

var arr = [ 1, 2, 3, 4 ];
var i = 0;

function log1( value, index, collection ) {
    console.log( '%s: %d', index, value );
    i += 1;
    if ( index === 0 && collection.length < 10 ) {
        collection.unshift( i+1 );
    }
}

forEachRight( arr, log1 );
/* =>
    3: 4
    2: 3
    1: 2
    0: 1
    0: 5
    0: 6
    0: 7
    0: 8
    0: 9
    0: 10
*/

function log2( value, index, collection ) {
    console.log( '%s: %d', index, value );
    collection.pop();
}

arr = [ 1, 2, 3, 4 ];

forEachRight( arr, log2 );
/* =>
    3: 4
    1: 2
*/

To set the function execution context, provide a thisArg.

function sum( value ) {
    this.sum += value;
    this.count += 1;
}

var arr = [ 1, 2, 3, 4 ];

var context = {
    'sum': 0,
    'count': 0
};

forEachRight( arr, sum, context );

var mean = context.sum / context.count;
// returns 2.5

Notes

• A collection may be either an Array, Typed Array, or an array-like Object (excluding strings and functions).

• The function differs from Array.prototype.forEach in the following ways:

  • The function returns the input collection.

  • The function does not skip undefined elements.

    function log( value, index ) {
        console.log( '%s: %s', index, value );
    }
    
    var arr = [ 1, , , 4 ];
    
    forEachRight( arr, log );
    /* =>
        3: 4
        2: undefined
        1: undefined
        0: 1
    */

  • The function provides limited support for dynamic collections (i.e., collections whose length changes during execution).

Examples

var isEven = require( '@stdlib/assert-is-even' ).isPrimitive;
var forEachRight = require( '@stdlib/utils-for-each-right' );

var arr;
var i;

function log( value, index, collection ) {
    console.log( '%s: %d', index, value );
    i += 1;
    if ( isEven( i ) ) {
        collection.pop();
    } else {
        collection.unshift( i+1 );
    }
}

arr = new Array( 100 );
for ( i = arr.length-1; i >= 0; i-- ) {
    arr[ i ] = i;
}

i = 0;
forEachRight( arr, log );

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See Also

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

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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

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License

See LICENSE.

Copyright

Copyright © 2016-2024. The Stdlib Authors.