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    Map.prototype.forEach()

    This is a new technology, part of the ECMAScript 2015 (ES6) standard .
    This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.

    The forEach() method executes a provided function once per each key/value pair in the Map object, in insertion order.

    Syntax

    myMap.forEach(callback[, thisArg])

    Parameters

    callback
    Function to execute for each element.
    thisArg
    Value to use as this when executing callback.

    Description

    The forEach method executes the provided callback once for each key of the map which actually exist. It is not invoked for keys which have been deleted. However, it is executed for values which are present but have the value undefined.

    callback is invoked with three arguments:

    • the element value
    • the element key
    • the Map object being traversed

    If a thisArg parameter is provided to forEach, it will be passed to callback when invoked, for use as its this value.  Otherwise, the value undefined will be passed for use as its this value.  The this value ultimately observable by callback is determined according to the usual rules for determining the this seen by a function.

    The range of elements processed by forEach is set before the first invocation of callback. Elements which are added to the Map object after the call to forEach begins will not be visited by callback. If existing elements of the Map object are changed, or deleted, their value as passed to callback will be the value at the time forEach visits them; elements that are deleted are not visited.

    forEach executes the callback function once for each element in the Map object; it does not return a value.

    Examples

    Printing the contents of a Map object

    The following code logs a line for each element in an Map object:

    function logMapElements(value, key, map) {
        console.log("m[" + key + "] = " + value);
    }
    Map([["foo", 3], ["bar", {}], ["baz", undefined]]).forEach(logMapElements);
    // logs:
    // "m[foo] = 3"
    // "m[bar] = [object Object]"
    // "m[baz] = undefined"
    

    Specifications

    Specification Status Comment
    ECMAScript 2015 (6th Edition, ECMA-262)
    The definition of 'Map.prototype.forEach' in that specification.
    Standard Initial definition.

    Browser compatibility

    Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
    Basic support 38 25.0 (25.0) 11 25 7.1
    Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
    Basic support Not supported 38 25.0 (25.0) Not supported Not supported 8

    See also

    Document Tags and Contributors

    Contributors to this page: fscholz, indolering, realityking
    Last updated by: fscholz,