Map.prototype.forEach()
The forEach()
method executes a provided function once per each key/value
pair in the Map
object, in insertion order.
Try it
Syntax
js
forEach(callbackFn)
forEach(callbackFn, thisArg)
Parameters
callbackFn
-
A function to execute for each entry in the map. The function is called with the following arguments:
value
-
Value of each iteration.
key
-
Key of each iteration.
map
-
The map being iterated.
thisArg
Optional-
A value to use as
this
when executingcallbackFn
.
Return value
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 entry's
value
- the entry's
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.
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.
Examples
Printing the contents of a Map object
The following code logs a line for each element in an Map
object:
js
function logMapElements(value, key, map) {
console.log(`map.get('${key}') = ${value}`);
}
new Map([
["foo", 3],
["bar", {}],
["baz", undefined],
]).forEach(logMapElements);
// Logs:
// "map.get('foo') = 3"
// "map.get('bar') = [object Object]"
// "map.get('baz') = undefined"
Specifications
Specification |
---|
ECMAScript Language Specification # sec-map.prototype.foreach |
Browser compatibility
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