5.3 KiB
reduceRight
Apply a function against an accumulator and each element in a collection and return the accumulated result, iterating from right to left.
Usage
var reduceRight = require( '@stdlib/utils/reduce-right' );
reduceRight( collection, initial, reducer[, thisArg ] )
Applies a function
against an accumulator and each element in a collection
and returns the accumulated result, iterating from right to left.
function sum( accumulator, value ) {
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var out = reduceRight( arr, 0, sum );
// returns 10
The reducer
function is provided four arguments:
accumulator
: accumulated valuevalue
: collection elementindex
: collection indexcollection
: 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 sum1( accumulator, value, index, collection ) {
i += 1;
if ( index === 0 && collection.length < 10 ) {
collection.unshift( i+1 );
}
return accumulator + value;
}
var out = reduceRight( arr, 0, sum1 );
// returns 55
function sum2( accumulator, value, index, collection ) {
collection.pop();
return accumulator + value;
}
arr = [ 1, 2, 3, 4 ];
out = reduceRight( arr, 0, sum2 );
// returns 6
To set the function execution context, provide a thisArg
.
function sum( accumulator, value ) {
this.count += 1;
return accumulator + value;
}
var arr = [ 1, 2, 3, 4 ];
var context = {
'count': 0
};
var out = reduceRight( arr, 0, sum, context );
// returns 10
var mean = out / context.count;
// returns 2.5
Notes
-
A
collection
may be either anArray
,Typed Array
, or an array-likeObject
(excludingstrings
andfunctions
). -
The function differs from
Array.prototype.reduceRight
in the following ways:-
The function requires an
initial
value for theaccumulator
. Theinitial
value is used during the first invocation of thereducer
function. -
The function does not skip the first element in the
collection
. -
The function does not skip
undefined
elements.function log( accumulator, value, index ) { console.log( '%s: %s', index, value ); return accumulator; } var arr = [ 1, , , 4 ]; var out = reduceRight( arr, 0, 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 reduceRight = require( '@stdlib/utils/reduce-right' );
var arr;
var s;
var i;
function sum( acc, value ) {
return acc + value;
}
arr = new Array( 1000 );
for ( i = 0; i < arr.length; i++ ) {
arr[ i ] = i;
}
s = reduceRight( arr, 0, sum );
console.log( s );