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

People also ask

What is the difference between the map () and forEach () in array prototype?

The returning value The first difference between map() and forEach() is the returning value. The forEach() method returns undefined and map() returns a new array with the transformed elements. Even if they do the same job, the returning value remains different.

What is the difference between forEach map and filter in Javascript?

The main difference between forEach and filter is that forEach just loop over the array and executes the callback but filter executes the callback and check its return value. If the value is true element remains in the resulting array but if the return value is false the element will be removed for the resulting array.

What is the difference between the map () and the forEach () method on the array prototype Linkedin?

Definition. The map() method creates a new array populated with the results of calling a provided function on every element in the calling array. The forEach() method executes a provided function once for each array element.

Is forEach or map faster?

There seems to be a very small difference between forEach() and map() with respect to speed. map() is faster, but these are so miniscule that it shouldn't affect your application's performance significantly. You can almost always use map() and other array methods like filter() and reduce() instead of using forEach().


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 if passed a callback function, it may do so).

References

Array.prototype.forEach() - JavaScript | MDN

Array.prototype.map() - JavaScript | MDN


  • 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() 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]

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');