Map.prototype.forEach()
The forEach()
method executes a
provided function once per each key/value pair in the Map
object, in
insertion order.
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Syntax
myMap.forEach(callback([value][, key][, map])[, thisArg])
Parameters
callback
-
Function to execute for each entry of
myMap
. It takes the following arguments:value
Optional- Value of each iteration.
key
Optional- Key of each iteration.
map
Optional- The map being iterated (
myMap
in the above Syntax box).
thisArg
Optional- Value to use as
this
when executingcallback
.
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:
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
Browser compatibility
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