Originally posted on my blog

JavaScript has some handy methods which help us iterate our arrays. The two most used for iteration is Array.prototype.map() and Array.prototype.forEach(), however, they remain a little bit unclear, especially for a beginner. Because they both do an iteration and output something. So, what is the difference?

• Definition
• 1. The returning value
• 2. Ability to chain other methods
• 3. Mutability
• 4. Performance Speed
• Final Thoughts

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.

1. 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.

const myAwesomeArray = [1, 2, 3, 4, 5]
myAwesomeArray.forEach(x => x * x)
//>>>>>>>>>>>>>return value: undefined

myAwesomeArray.map(x => x * x)
//>>>>>>>>>>>>>return value: [1, 4, 9, 16, 25]

2. Ability to chain other methods

The second difference between these array methods is the fact that map() is chainable, that's mean, you can attach reduce(), sort(), filter() etc. after performing a map() method to an array. That's something you can't do with forEach() because as you might guess it returns undefined.

const myAwesomeArray = [1, 2, 3, 4, 5]
myAwesomeArray.forEach(x => x * x).reduce((total, value) => total + value)
//>>>>>>>>>>>>> Uncaught TypeError: Cannot read property 'reduce' of undefined
myAwesomeArray.map(x => x * x).reduce((total, value) => total + value)
//>>>>>>>>>>>>>return value: 55

3. Mutability

According to MDN documentation:

forEach() does not mutate the array on which it is called. (However, callback may do so).

map() does not mutate the array on which it is called (although callback, if invoked, may do so).

JavaScript is weird.

Here, we see a very similar definition, and, we all know that they both receive a callback as an argument, so, which one relies on immutability?

Well, in my opinion, this definition is not clear though. And to know which does not mutate the original array, we first have to check how these two methods work.

The map() method returns an entirely new array with transformed elements and the same amount of data. In the case of forEach(), even if it returns undefined, it will mutate the original array with the callback.

Therefore, we see clearly that map() relies on immutability and forEach() is a mutator method.

4. Performance Speed

Regarding performance speed, they are a little bit different. But, does it matter? Well, it depends on various things like the computer, the amount of data, etc. You can check it on your own with this example below or with jsPerf to see which is faster.

const myAwesomeArray = [1, 2, 3, 4, 5]

const startForEach = performance.now()
myAwesomeArray.forEach(x => (x + x) * 10000000000)
const endForEach = performance.now()
console.log(`Speed [forEach]: ${endForEach - startForEach} miliseconds`)

const startMap = performance.now()
myAwesomeArray.map(x => (x + x) * 10000000000)
const endMap = performance.now()
console.log(`Speed [map]: ${endMap - startMap} miliseconds`)

Final Thoughts

As always, the choice between map() and forEach() will depend on the use case. If you plan to change, alternate or use the data, it's preferable to pick map(), because it returns a new array with the transformed data. But, if you won't need the returned array, don't use map(), instead use forEach() or even a for loop.

Hopefully, this post makes clear the differences between these two methods. If there are more differences, please share them in the comment section, otherwise thanks for reading it.

Photo by Franck V. on Unsplash