JavaScript：.forEach() 和.map() 之间的区别

Question

I know that there were a lot of topics like this. And I know the basics: .forEach() operates on original array and .map() on the new one.

In my case:

function practice (i){
    return i+1;
};

var a = [ -1, 0, 1, 2, 3, 4, 5 ];
var b = [ 0 ];
var c = [ 0 ];
console.log(a);
b = a.forEach(practice);
console.log("=====");
console.log(a);
console.log(b);
c = a.map(practice);
console.log("=====");
console.log(a);
console.log(c);

And this is output:

[ -1, 0, 1, 2, 3, 4, 5 ]
=====
[ -1, 0, 1, 2, 3, 4, 5 ]
undefined
=====
[ -1, 0, 1, 2, 3, 4, 5 ]
[ 0, 1, 2, 3, 4, 5, 6 ]

I can't understand why using practice changes value of b to undefined .
I'm sorry if this is silly question, but I'm quite new in this language and answers I found so far didn't satisfy me.

Answer 1

They are not one and the same. Let me explain the difference.

forEach : This iterates over a list and applies some operation with side effects to each list member (example: saving every list item to the database) and does not return anything.

map : This iterates over a list, transforms each member of that list, and returns another list of the same size with the transformed members (example: transforming list of strings to uppercase). It does not mutate the array on which it is called (although the callback function may do so).

References

Array.prototype.forEach() - JavaScript | MDN

Array.prototype.map() - JavaScript | MDN

Answer 2

• Array.forEach “executes a provided function once per array element.”

• Array.map “creates a new array with the results of calling a provided function on every element in this array.”

So, forEach doesn't actually return anything. It just calls the function for each array element and then it's done. So whatever you return within that called function is simply discarded.

On the other hand, map will similarly call the function for each array element but instead of discarding its return value, it will capture it and build a new array of those return values.

This also means that you could use map wherever you are using forEach but you still shouldn't do that so you don't collect the return values without any purpose. It's just more efficient to not collect them if you don't need them.

Answer 3

	forEach()	map()
Functionality	Performs given operation on each element of the array	Performs given "transformation" on a "copy" of each element
Return value	Returns undefined	Returns new array with transformed elements, leaving back original array unchanged.
Preferrable usage scenario and example	Performing non-tranformation like processing on each element. For example, saving all elements in the database.	Obtaining array containing output of some processing done on each element of the array. For example, obtaining array of lengths of each string in the array

forEach() example

 chars = ['Hello' , 'world!!!'] ; var retVal = chars.forEach(function(word){ console.log("Saving to db: " + word) }) console.log(retVal) //undefined

map() example

 chars = ['Hello' , 'world!!!'] ; var lengths = chars.map(function(word){ return word.length }) console.log(lengths) //[5,8]

Answer 4

The main difference that you need to know is .map() returns a new array while .forEach() doesn't. That is why you see that difference in the output. .forEach() just operates on every value in the array.

Read up:

• Array.prototype.forEach() - JavaScript | MDN
• Array.prototype.map() - JavaScript | MDN

You might also want to check out: - Array.prototype.every() - JavaScript | MDN

Answer 5

Performance Analysis For loops performs faster than map or foreach as number of elements in a array increases.

 let array = []; for (var i = 0; i < 20000000; i++) { array.push(i) } console.time('map'); array.map(num => { return num * 4; }); console.timeEnd('map'); console.time('forEach'); array.forEach((num, index) => { return array[index] = num * 4; }); console.timeEnd('forEach'); console.time('for'); for (i = 0; i < array.length; i++) { array[i] = array[i] * 2; } console.timeEnd('for');

Answer 6

forEach: If you want to perform an action on the elements of an Array and it is same as you use for loop. The result of this method does not give us an output buy just loop through the elements.

map: If you want to perform an action on the elements of an array and also you want to store the output of your action into an Array. This is similar to for loop within a function that returns the result after each iteration.

Hope this helps.

Answer 7

map returns a new array.

forEach has no return value.

That's the heart of the difference. Most of the other answers here say effectively that, but in a much more convoluted way.

Answer 8

The difference lies in what they return. After execution:

arr.map()

returns an array of elements resulting from the processed function; while:

arr.forEach()

returns undefined.

Answer 9

forEach() :

return value : undefined

originalArray : not modified after the method call

newArray is not created after the end of method call.

map() :

return value : new Array populated with the results of calling a provided function on every element in the calling array

originalArray : not modified after the method call

newArray is created after the end of method call.

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.

Answer 10

Diffrence between Foreach & map :

Map() : If you use map then map can return new array by iterating main array.

Foreach() : If you use Foreach then it can not return anything for each can iterating main array.

useFul link : use this link for understanding diffrence

https://codeburst.io/javascript-map-vs-foreach-f38111822c0f

Answer 11

Difference between forEach() & map()

forEach() just loop through the elements. It's throws away return values and always returns undefined.The result of this method does not give us an output .

map() loop through the elements allocates memory and stores return values by iterating main array

Example:

   var numbers = [2,3,5,7];

   var forEachNum = numbers.forEach(function(number){
      return number
   })
   console.log(forEachNum)
   //output undefined

   var mapNum = numbers.map(function(number){
      return number
   })
   console.log(mapNum)
   //output [2,3,5,7]

map() is faster than forEach()

Answer 12

one of the shuttle difference not mentioned here is that forEach() can loop over static (not live) NodeList while map() cannot

//works perfectly
      document.querySelectorAll('.score').forEach(element=>console.log(element));
    
    
  //Uncaught TypeError: document.querySelectorAll(...).map is not a function        
      document.querySelectorAll('.score').map(element=>console.log(element));

Answer 13

One thing to point out is that both methods skips uninitialized values, but map keeps them in the returned array.

var arr = [1, , 3];

arr.forEach(function(element) {
    console.log(element);
});
//Expected output: 1 3

console.log(arr.map(element => element));
//Expected output: [1, undefined, 3];

Answer 14

The main difference between forEach and map method is that forEach method just loop through the array and doesn't return anything.

let arr = [1,2,3,4];
let newArr = arr.forEach((x) => x);
console.log(newArr); // prints undefined

While, map returns a new array.

let arr = [1,2,3,4];
let newArr = arr.map((x) => x);
console.log(newArray); // [1,2,3,4]

We can use map to create a deep copy of an array.

Answer 15

const arr = [...Array(100000000).keys()];

console.time("for");
for (let i = 0; i < arr.length; i++) {}
console.timeEnd("for");

console.time("while");
let j = 0;
while (j < arr.length) {
  j++;
}
console.timeEnd("while");

console.time("dowhile");
let k = 0;
do {
  k++;
} while (k < arr.length);
console.timeEnd("dowhile");

console.time("forEach");
arr.forEach((element) => {});
console.timeEnd("forEach");

VM35:6 for: 45.998046875 ms

VM35:13 while: 154.581787109375 ms

VM35:20 dowhile: 141.97216796875 ms

VM35:24 forEach: 776.469970703125 ms

Answer 16

Performance Analysis (again - not very scientific)
In my experience sometime .map() can be faster than .foreach()

 let rows = []; for (let i = 0; i < 10000000; i++) { // console.log("here", i) rows.push({ id: i, title: 'ciao' }); } const now1 = Date.now(); rows.forEach(row => { if (!row.event_title) { row.event_title = `no title ${row.event_type}`; } }); const now2 = Date.now(); rows = rows.map(row => { if (!row.event_title) { row.event_title = `no title ${row.event_type}`; } return row; }); const now3 = Date.now(); const time1 = now2 - now1; const time2 = now3 - now2; console.log('forEach time', time1); console.log('.map time', time2);

On my macbook pro (late 2013)

• forEach time 1909
• .map time 444

Answer 17

.map and .forEach will do just about then same thing, until you start operating on arrays with millions of elements. .map will create another collection with the same size (and possibly type, depending on the array species) which could use up a LOT of memory. .forEach will not do this.

Answer 18

Map implicitly returns while forEach does not.

This is why when you're coding a JSX application, you almost always use map instead of forEach to display content in React .