Dive Into JavaScript

JavaScript Array reduce

The JavaScript Array reduce method iteratively reduces an array to a single value using a callback function.

Syntax

array.reduce(callback [, initialValue])

Parameters

callback

The callback function to execute for each element

initialValue

The initial value to use as the first argument to the first call of the callback

Cross-browser Array reduce

For browsers that don't support the Array reduce method, here's an alternative implementation:

/**
 * Copyright (c) Mozilla Foundation http://www.mozilla.org/
 * This code is available under the terms of the MIT License
 */
if (!Array.prototype.reduce) {
    Array.prototype.reduce = function(fun /*, initial*/) {
        var len = this.length >>> 0;
        if (typeof fun != "function") {
            throw new TypeError();
        }

        // no value to return if no initial value and an empty array
        if (len == 0 && arguments.length == 1)
            throw new TypeError();

        var i = 0;
        if (arguments.length >= 2) {
            var rv = arguments[1];
        }
        else {
            do {
                if (i in this) {
                    var rv = this[i++];
                    break;
                }

                // if array contains no values, no initial value to return
                if (++i >= len) {
                    throw new TypeError();
                }
            }
            while (true);
        }

        for (; i < len; i++) {
            if (i in this) {
                rv = fun.call(null, rv, this[i], i, this);
            }
        }

        return rv;
    };
}

See also

• The JavaScript Array Object
• JavaScript Array every
• JavaScript Array filter
• JavaScript Array forEach
• JavaScript Array map
• JavaScript Array reduceRight
• JavaScript Array some