Set.prototype.forEach() - JavaScript 编辑

The forEach() method executes a provided function once for each value in the Set object, in insertion order.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

mySet.forEach(callback[, thisArg])

Parameters

callback

Function to execute for each element, taking three arguments:

currentValue, currentKey

The current element being processed in the Set. As there are no keys in Set, the value is passed for both arguments.

set

The Set object which forEach() was called upon.

thisArg

Value to use as this when executing callback.

Return value

undefined.

Description

The forEach() method executes the provided callback once for each value which actually exists in the Set object. It is not invoked for values 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 Set object being traversed

There are no keys in Set objects, however, so the first two arguments are both values contained in the Set. This is to make it consistent with other forEach() methods for Map and Array.

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.

Each value is visited once, except in the case when it was deleted and re-added before forEach() has finished. callback is not invoked for values deleted before being visited. New values added before forEach() has finished will be visited.

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

Examples

Logging the contents of a Set object

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

function logSetElements(value1, value2, set) {
    console.log('s[' + value1 + '] = ' + value2);
}

new Set(['foo', 'bar', undefined]).forEach(logSetElements);

// logs:
// "s[foo] = foo"
// "s[bar] = bar"
// "s[undefined] = undefined"

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'Set.prototype.forEach' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• Array.prototype.forEach()
• Map.prototype.forEach()