JavaScript: Difference between .forEach() and .map()
JavascriptArraysLoopsForeachJavascript Problem Overview
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.
Javascript Solutions
Solution 1 - Javascript
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
Solution 2 - Javascript
-
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.
Solution 3 - Javascript
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]
Solution 4 - Javascript
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:
You might also want to check out:
Solution 5 - Javascript
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');
Solution 6 - Javascript
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.
Solution 7 - Javascript
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.
Solution 8 - Javascript
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.
Solution 9 - Javascript
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.
Solution 10 - Javascript
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
Solution 11 - Javascript
> 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()
Solution 12 - Javascript
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));
Solution 13 - Javascript
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];
Solution 14 - Javascript
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
Solution 15 - Javascript
.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.
Solution 16 - Javascript
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.