forEachRight

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

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 );