JavaScript Array prototype reduce

The reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.

Syntax

arr.reduce(callback[, initialValue])

Parameters

• callback Function to execute on each value in the array, taking four arguments:

  • previousValue The value previously returned in the last invocation of the callback, or initialValue, if supplied. (See below.)

  • currentValue The current element being processed in the array.

  • currentIndex The index of the current element being processed in the array.

  • array The array reduce was called upon.

• initialValue Optional. Value to use as the first argument to the first call of the callback.

MDN link | MSDN link

Returns

The accumulated result from the last call to the callback function.

Description

Calls the specified callback function for all the elements in an array. The return value of the callback function is the accumulated result, and is provided as an argument in the next call to the callback function.

Remarks

If an initialValue is provided, the reduce method calls the callback function one time for each element present in the array, in ascending index order. If an initialValue is not provided, the reduce method calls the callback function on each element, starting with the second element.

The return value of the callback function is provided as the previousValue argument on the next call to the callback function. The return value of the last call to the callback function is the return value of the reduce method.

The callback function is not called for missing elements of the array.

Examples

var total = [0, 1, 2, 3].reduce(function(a, b) {
  return a + b;
});
// total == 6

var flattened = [0, 1], [2, 3], [4, 5](/thelastmile/FreeCodeCamp/wiki/0,-1],-[2,-3],-[4,-5).reduce(function(a, b) {
  return a.concat(b);
}, []);
// flattened is [0, 1, 2, 3, 4, 5]

// Define the callback function.
function appendCurrent (previousValue, currentValue) {
    return previousValue + "::" + currentValue;
    }

// Create an array.
var elements = ["abc", "def", 123, 456];

// Call the reduce method, which calls the callback function
// for each array element.
var result = elements.reduce(appendCurrent);

document.write(result);

// Output:
//  abc::def::123::456