Source @ github

Array.reduce

reduce — Loops thru the array, and for each value the result of the last callback is made available.

Description

Array.reduce(function callback, [mixed first])

Parameters

Name Description Type Default Optional
callback See callback below. function No
first See first below. mixed Yes
callback

The following parameters is passed to the callback.

previous

The value of the current index.

value

The value of the current index.

index

The current index.

array

The array.

first

The value of previous for the first iteration.

If not provided the first loop is skipped.

Return values

Returns a new array with the return value from the callback.

Examples

Example #1 – reduce example
var array = [2, 4, 8];

var result = array.reduce(function () {
	console.log(this);
	return true;
});

console.log(result);  // true
console.log(array);   // [2, 4, 8]
var arr = [2, 4, 8];

var result = arr.reduce(function (previous, value, index, array) {
	console.log(previous, value, index, array);
	return true;
});

console.log(result);  // true
console.log(arr);     // [2, 4, 8]

Passing in a custom object for this.

var array = [2, 4, 8];

var result = array.reduce(function () {
	console.log(this);
	return true;
}, document.location);

console.log(result);  // true
console.log(array);   // [2, 4, 8]

False for 4

var array = [2, 4, 8];

var result = array.reduce(function (value) {
	if (value === 4) {
		return false;
	}
	return true;
});

console.log(result);  // true
console.log(array);   // [2, 4, 8]

Return false for all.

var array = [2, 4, 8];

var result = array.reduce(function (value) {
	console.log(value);
	return false;
});

console.log(result);  // false
console.log(array);   // [2, 4, 8]

Return a new array with values doubled of the input.

var array = [1, 2, 3, 4, 5];

var result = array.reduce(function (previous, value) {
	var sum = previous + value;
	console.log(previous, value, sum);
	return sum;
});

console.log(result);  // 15
var array = [1, 2, 3, 4, 5];

var result = array.reduce(function (previous, value) {
	var sum = previous + value;
	console.log(previous, value, sum);
	return sum;
}, 0);

console.log(result);  // 15
var array = [1, 2, 3, 4, 5];

var result = array.reduce(function (previous, value) {
	var sum = previous * value;
	console.log(previous, value, sum);
	return sum;
}, 10);

console.log(result);  // 1200

External references