Array.prototype.map()
The map()
method creates
a new array populated with the results of calling a provided function on
every element in the calling array.
Syntax
// Arrow function
map((element) => { /* ... */ })
map((element, index) => { /* ... */ })
map((element, index, array) => { /* ... */ })
// Callback function
map(callbackFn)
map(callbackFn, thisArg)
// Inline callback function
map(function(element) { /* ... */ })
map(function(element, index) { /* ... */ })
map(function(element, index, array){ /* ... */ })
map(function(element, index, array) { /* ... */ }, thisArg)
Parameters
callbackFn
-
Function that is called for every element of
arr
. Each timecallbackFn
executes, the returned value is added tonewArray
.The function is called with the following arguments:
element
-
The current element being processed in the array.
index
-
The index of the current element being processed in the array.
array
-
The array
map
was called upon.
thisArg
Optional-
Value to use as
this
when executingcallbackFn
.
Return value
A new array with each element being the result of the callback function.
Description
map
calls a provided callbackFn
function
once for each element in an array, in order, and constructs a new array
from the results. callbackFn
is invoked only for indexes of the
array which have assigned values (including undefined
).
It is not called for missing elements of the array; that is:
- indexes that have never been set;
- indexes which have been deleted.
When not to use map()
Since map
builds a new array, using it when you aren't using the returned
array is an anti-pattern; use forEach
or
for...of
instead.
You shouldn't be using map
if:
- you're not using the array it returns; and/or
- you're not returning a value from the callback.
Parameters in Detail
callbackFn
is invoked with three arguments: the value of the
element, the index of the element, and the array object being mapped.
If a thisArg
parameter is provided, it will be used as callback's
this
value. Otherwise, the value undefined
will be used as
its this
value. The this
value ultimately observable by
callbackFn
is determined according to
the usual rules for determining the this
seen by a function.
map
does not mutate the array on which it is called (although
callbackFn
, if invoked, may do so).
The range of elements processed by map
is set before the first invocation
of callbackFn
. Elements which are assigned to indexes already visited, or to indexes
outside the range, will not be visited by callbackFn
.
If existing elements of the array are changed after the call to map
, their
value will be the value at the time callbackFn
visits them.
Elements that are deleted after the call to map
begins and before being
visited are not visited.
Warning: Concurrent modification of the kind described in the previous paragraph frequently leads to hard-to-understand code and is generally to be avoided (except in special cases).
Due to the algorithm defined in the specification, if the array which map
was called upon is sparse, resulting array will also be sparse keeping same indices
blank.
Examples
Mapping an array of numbers to an array of square roots
The following code takes an array of numbers and creates a new array containing the square roots of the numbers in the first array.
const numbers = [1, 4, 9];
const roots = numbers.map((num) => Math.sqrt(num));
// roots is now [1, 2, 3]
// numbers is still [1, 4, 9]
Using map to reformat objects in an array
The following code takes an array of objects and creates a new array containing the newly reformatted objects.
const kvArray = [{ key: 1, value: 10 },
{ key: 2, value: 20 },
{ key: 3, value: 30 }];
const reformattedArray = kvArray.map(({ key, value}) => ({ [key]: value }));
// reformattedArray is now [{1: 10}, {2: 20}, {3: 30}],
// kvArray is still:
// [{key: 1, value: 10},
// {key: 2, value: 20},
// {key: 3, value: 30}]
Mapping an array of numbers using a function containing an argument
The following code shows how map
works when a function requiring one
argument is used with it. The argument will automatically be assigned from each element
of the array as map
loops through the original array.
const numbers = [1, 4, 9];
const doubles = numbers.map((num) => num * 2);
// doubles is now [2, 8, 18]
// numbers is still [1, 4, 9]
Using map generically
This example shows how to use map on a String
to get an array of bytes in
the ASCII encoding representing the character values:
const map = Array.prototype.map;
const charCodes = map.call('Hello World', (x) => x.charCodeAt(0));
// charCodes now equals [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100]
Using map generically querySelectorAll
This example shows how to iterate through a collection of objects collected by
querySelectorAll
. This is because querySelectorAll
returns a
NodeList
(which is a collection of objects).
In this case, we return all the selected option
s' values on the screen:
const elems = document.querySelectorAll('select option:checked');
const values = Array.prototype.map.call(elems, ({ value }) => value);
An easier way would be the Array.from()
method.
Tricky use case
It is common to use the callback with one argument (the element being traversed). Certain functions are also commonly used with one argument, even though they take additional optional arguments. These habits may lead to confusing behaviors.
Consider:
['1', '2', '3'].map(parseInt);
While one might expect [1, 2, 3]
, the actual result is
[1, NaN, NaN]
.
parseInt
is often used with one argument, but takes two. The first is an
expression and the second is the radix to the callback function,
Array.prototype.map
passes 3 arguments:
- the element
- the index
- the array
The third argument is ignored by parseInt
—but not the second
one! This is the source of possible confusion.
Here is a concise example of the iteration steps:
// parseInt(string, radix) -> map(parseInt(value, index))
/* first iteration (index is 0): */ parseInt("1", 0); // 1
/* second iteration (index is 1): */ parseInt("2", 1); // NaN
/* third iteration (index is 2): */ parseInt("3", 2); // NaN
Then let's talk about solutions.
const returnInt = (element) => parseInt(element, 10);
['1', '2', '3'].map(returnInt); // [1, 2, 3]
// Actual result is an array of numbers (as expected)
// Same as above, but using the concise arrow function syntax
['1', '2', '3'].map((str) => parseInt(str)); // [1, 2, 3]
// A simpler way to achieve the above, while avoiding the "gotcha":
['1', '2', '3'].map(Number); // [1, 2, 3]
// But unlike parseInt(), Number() will also return a float or (resolved) exponential notation:
['1.1', '2.2e2', '3e300'].map(Number); // [1.1, 220, 3e+300]
// For comparison, if we use parseInt() on the array above:
['1.1', '2.2e2', '3e300'].map((str) => parseInt(str)); // [1, 2, 3]
One alternative output of the map method being called with parseInt
as a
parameter runs as follows:
const strings = ['10', '10', '10'];
const numbers = strings.map(parseInt);
console.log(numbers);
// Actual result of [10, NaN, 2] may be unexpected based on the above description.
Mapped array contains undefined
When undefined
or nothing is returned:
const numbers = [1, 2, 3, 4];
const filteredNumbers = numbers.map((num, index) => {
if (index < 3) {
return num;
}
});
// index goes from 0, so the filterNumbers are 1,2,3 and undefined.
// filteredNumbers is [1, 2, 3, undefined]
// numbers is still [1, 2, 3, 4]
Specifications
Specification |
---|
ECMAScript Language Specification # sec-array.prototype.map |
Browser compatibility
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