JavaScript: Difference between .forEach() and .map()
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
Is there a difference between foreach and map?
Different.
foreach iterates over a list and performs some operation with side effects to each list member (such as saving each one to the database for example)
map iterates over a list, transforms each member of that list, and returns another list of the same size with the transformed members (such as converting a list of strings to uppercase)
What is the big difference between forEach and map?
The difference is that forEach
"iterates over an array" and map
"iterates over an array and returns the result"
var numbers = [1, 4, 9];
var roots = numbers.forEach(Math.sqrt);
// roots is undefined, numbers is still [1, 4, 9]
var roots = numbers.map(Math.sqrt);
// roots is [1, 2, 3], numbers is still [1, 4, 9]
What's so good about map
? It keeps your code small and smart because you don't need to define anonymous functions for built-in functions.
var numbers = [1, 4, 9];
var roots = [];
numbers.forEach(function(val){ roots.push( Math.sqrt(val) ); });
// roots is now [1, 2, 3], numbers is still [1, 4, 9]
The value of returning the result (no pun intended) is proven in your existing example...
var kvArray = [{key:1, value:10}, {key:2, value:20}, {key:3, value: 30}];
var reformattedArray = kvArray.map(function(obj){ // <---------------- map
var rObj = {};
rObj[obj.key] = obj.value;
return rObj;
});
// reformattedArray is now [{1:10}, {2:20}, {3:30}],
// kvArray is still [{key:1, value:10}, {key:2, value:20}, {key:3, value: 30}]
var kvArray = [{key:1, value:10}, {key:2, value:20}, {key:3, value: 30}];
var reformattedArray = kvArray.forEach(function(obj){ // <---------------- forEach
var rObj = {};
rObj[obj.key] = obj.value;
return rObj;
});
// reformattedArray is now undefined,
// kvArray is still [{key:1, value:10}, {key:2, value:20}, {key:3, value: 30}]
What is the difference between .map, .every, and .forEach?
The difference is in the return values.
.map()
returns a new Array of objects created by taking some action on the original item.
.every()
returns a boolean - true if every element in this array satisfies the provided testing function. An important difference with .every()
is that the test function may not always be called for every element in the array. Once the testing function returns false for any element, no more array elements are iterated. Therefore, the testing function should usually have no side effects.
.forEach()
returns nothing - It iterates the Array performing a given action for each item in the Array.
Read about these and the many other Array iteration methods at MDN.
Difference between map, filter vs for and forEach and what would be the advantages of using map and filter over For and forEach loops?
Other answer's actually explains the reasons. Here is a simple bench marking example. Use the link to run the test.
http://jsben.ch/cz0Cg
Posting the results in case if link expires.
JS For Loop
var foo1 = ['spray', 'limit', 'elite', 'exuberant', 'destruction','present'];
for(var i=0;i<foo1.length;i++){
console.log(foo1[i]);
}
Execution time : 677 ms
JS Map
var foo2 = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];
foo2.map((v,i)=>console.log(v));
Execution time : 686 ms
JS Filter
var foo3 = ['spray', 'limit', 'elite', 'exuberant', 'destruction', 'present'];
const result = foo3.filter(word => word.length > 6);
console.log(result);
Execution time : 259 ms
Execution Time given is when I tested, it is subject to change.
array.forEach.call vs array.map.call
forEach operates on the original array elements. (If you want just iterate over all element you should use forEach)
map is running through your array, applying a function to each element, and emitting the result as a new array. (if you want to apply some changes to each element you should use map)
forEach and map in JavaScript
Neither function, map
or forEach
mutate the original array. The difference here is that map
returns a new array while forEach
doesn't. Which is why you have
a = a.map(el => {
You assign the array returned by the map
back to the variable a
. With a forEach
the return values inside the callback are not used and there is no new array to work with.
const array = [1, 2, 3, 4, 5]
const square = number => number * number
// logs [1, 4, 9, 16, 25]
console.log(array.map(square))
// logs undefined
console.log(array.forEach(square))
What use does the JavaScript forEach method have (that map can't do)?
The essential difference between map
and forEach
in your example is that forEach
operates on the original array elements, whereas map
explicitly returns a new array as a result.
With forEach
you are taking some action with -- and optionally changing -- each element in the original array. The forEach
method runs the function you provide for each element, but returns nothing (undefined
). On the other hand, map
walks through the array, applies a function to each element, and emits the result as a new array.
The "side effect" with forEach
is that the original array is being changed. "No side effect" with map
means that, in idiomatic usage, the original array elements are not changed; the new array is a one-to-one mapping of each element in the original array -- the mapping transform being your provided function.
The fact that there's no database involved does not mean that you won't have to operate on data structures, which, after all, is one of the essences of programming in any language. As for your last question, your array can contain not only numbers, but objects, strings, functions, etc.
Difference in performance (map(), forEach(), for())
By using an async
function inside .map
and .forEach
you fire and forget the asynchronous action, which means that you won't know when it finished. The async
function does however return a Promise, and if you use .map
you could collect them in an array, call Promise.all
on that and await that:
await Promise.all(result.map(async (n) => {
console.log("inside map ", counter);
await this.processImage(n, counter++);
counter++; // why increase counter twice?
}));
// all processings are guaranteed to be done
This will execute all the processing in parallel, which is probably (way) faster than processing sequentially which you'd do with the for
loop. Using a .forEach
you aren't able to wait for all results to arrive, and therefore you probably don't want to use it in most cases.
If you arent doing asynchronous things, for
and .forEach
would behave nearly equal, except for arrays with empty slots (.forEach
skips empty slots):
for(const el of Array(3))
console.log(el); // logs undefined three times
Array(3).forEach(console.log) // silence
.map
behaves like .forEach
just that it builds up an array with the returned values, just as you said.
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