JavaScript: Difference between .forEach() and .map()

• 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.

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]

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

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');

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.

Conclusion:

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.

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.

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.

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.

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()

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

.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.

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];

One of the subtle differences not mentioned here is that forEach() can loop over a 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));

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