Ibrahima Ndaw

Posted on Jan 9, 2020 • Edited on Apr 21, 2020 • Originally published at ibrahima-ndaw.com

15 must-know JavaScript array methods in 2020

#javascript #beginners

Originally posted on my blog

In JavaScript, an array is a special variable that is used to store different elements. It has some built-in properties and methods we can use to add, remove, iterate, or manipulate data following our needs. And knowing JavaScript array methods can lift your skills as a developer.

In this post, we're going to see 15 array methods that can help you manipulate your data properly.

1. some()
2. reduce()
3. every()
4. map()
5. flat()
6. filter()
7. forEach()
8. findIndex()
9. find()
10. sort()
11. concat()
12. fill()
13. includes()
14. reverse()
15. flatMap()

Notice that mostly in this post, we'll simplify the function passed as parameter.

// Instead of using this way
myAwesomeArray.some(test => {
  if (test === "d") {
    return test
  }
})
// We'll use the shorter one
myAwesomeArray.some(test => test === "d")

1. some()

This method tests the array with a function passed as a parameter. It will return true if at least one element matches the test and false for the opposite.

const myAwesomeArray = ["a", "b", "c", "d", "e"]

myAwesomeArray.some(test => test === "d")
//-------> Output : true

2. reduce()

This method receives a function which has an accumulator and a value as an argument. It applies the function to the accumulator and each value in the array to return at the end just a single value.

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

myAwesomeArray.reduce((total, value) => total * value)
// 1 * 2 * 3 * 4 * 5
//-------> Output = 120

3. every()

This method tests the array with a function passed as a parameter. It will return true if each element of the array match the test and false for the opposite.

const myAwesomeArray = ["a", "b", "c", "d", "e"]

myAwesomeArray.every(test => test === "d")
//-------> Output : false

const myAwesomeArray2 = ["a", "a", "a", "a", "a"]

myAwesomeArray2.every(test => test === "a")
//-------> Output : true

4. map()

This method receives a function as a parameter. And return a new array that contains an image of each element of the array. It will always return the same amount of items.

const myAwesomeArray = [5, 4, 3, 2, 1]
myAwesomeArray.map(x => x * x)

//-------> Output : 25
//                  16
//                  9
//                  4
//                  1

5. flat()

This method creates a new array that contains the elements holden on the sub-array and flat it into the new array. Notice that, this method will go only one level depth.

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

myAwesomeArray.flat()
//-------> Output : [1, 2, 3, 4, 5]

6. filter()

This method receives a function as a parameter. And return a new array that contains all the elements of the array for which the filtering function passed as argument returns true.

const myAwesomeArray = [
  { id: 1, name: "john" },
  { id: 2, name: "Ali" },
  { id: 3, name: "Mass" },
  { id: 4, name: "Mass" },
]

myAwesomeArray.filter(element => element.name === "Mass")
//-------> Output : 0:{id: 3, name: "Mass"},
//                  1:{id: 4, name: "Mass"}

7. forEach()

This method applies a function to each element of the array.

const myAwesomeArray = [
  { id: 1, name: "john" },
  { id: 2, name: "Ali" },
  { id: 3, name: "Mass" },
]

myAwesomeArray.forEach(element => console.log(element.name))
//-------> Output : john
//                  Ali
//                  Mass

8. findIndex()

This method receives a function as a parameter and will apply it to the array. It returns the index of an element found and which satisfies the test function passed as an argument or -1 if none satisfies it.

const myAwesomeArray = [
  { id: 1, name: "john" },
  { id: 2, name: "Ali" },
  { id: 3, name: "Mass" },
]

myAwesomeArray.findIndex(element => element.id === 3)
//-------> Output : 2

myAwesomeArray.findIndex(element => element.id === 7)
//-------> Output : -1

9. find()

This method receives a function as an argument and will apply it to the array. It returns the value of an element found in the array and which satisfies the test function. Otherwise, it returns undefined.

const myAwesomeArray = [
  { id: 1, name: "john" },
  { id: 2, name: "Ali" },
  { id: 3, name: "Mass" },
]

myAwesomeArray.find(element => element.id === 3)
//-------> Output : {id: 3, name: "Mass"}

myAwesomeArray.find(element => element.id === 7)
//-------> Output : undefined

10. sort()

This method receives a function as a parameter. It sorts the elements of an array and returns it.

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

// Sort from smallest to largest
myAwesomeArray.sort((a, b) => a - b)
//-------> Output : [1, 2, 3, 4, 5]

// Sort from largest to smallest
myAwesomeArray.sort((a, b) => b - a)
//-------> Output : [5, 4, 3, 2, 1]

11. concat()

This method will merge two or more arrays/values by concatenating it. It returns a new array with the elements.

const myAwesomeArray = [1, 2, 3, 4, 5]
const myAwesomeArray2 = [10, 20, 30, 40, 50]
myAwesomeArray.concat(myAwesomeArray2)
//-------> Output : [1, 2, 3, 4, 5, 10, 20, 30, 40, 50]

12. fill()

This method fills all the elements of a given array with the same value, from a start index (default 0) to an end index (default array.length).

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

// The first argument (0) is the value
// The second argument (1) is the starting index
// The third argument (3) is the ending index
myAwesomeArray.fill(0, 1, 3)
//-------> Output : [1, 0, 0, 4, 5]

13. includes()

This method will return true if the array contains a certain element, and false if not.

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

myAwesomeArray.includes(3)
//-------> Output : true

myAwesomeArray.includes(8)
//-------> Output : false

14. reverse()

This method reverses an array. The first element becomes the last, and the last element will be the first.

const myAwesomeArray = ["e", "d", "c", "b", "a"]

myAwesomeArray.reverse()
//-------> Output : ['a', 'b', 'c', 'd', 'e']

15. flatMap()

The method applies a function to each element of the array and then flatten the result into an array. It combines flat() and map() in one function.

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

myAwesomeArray.flatMap(arr => arr * 10)
//-------> Output : [10, 20, 30, 40, 50]

// With .flat() and .map()
myAwesomeArray.flat().map(arr => arr * 10)
//-------> Output : [10, 20, 30, 40, 50]

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Top comments (71)

Marco Damaceno • Jan 9 '20

This post could be called just "15 must-know JavaScript array methods". No need the "in 2020".

Idan Arye • Jan 9 '20

I don't think these methods were really necessary in 2019, but now? Can't do without them.

George Jempty • Jan 9 '20

I'd say they were necessary in 2018 and possibly even earlier. All these "in 2020" articles are clickbait in my opinion.

Ibrahima Ndaw • Jan 10 '20

This is your opinion, and I fully respect it. However, I've added "in 2020", to just make this article easy to find for beginners. And also knowing these methods is relevant for 2020, in my opinion, because most of them introduce you to functional programming.

Suraj Mandal • Dec 23 '20

I see that I tend to click more on articles which have "in 2020" in the title so I guess it works since this is what I googled for :)

Jochem Stoel • Jan 9 '20

@idan haha that is brilliant response.

Ibrahima Ndaw • Jan 9 '20

I totally agree. We hear more and more functional programming in JavaScript world. And using some of these methods makes sense because they manipulate the array in an immutable way.

Ibrahima Ndaw • Jan 9 '20

I think "in 2020" is more fun. No, I'm kidding. I will update the title next week to not limit it in "2020" only. Thanks for your comment

Rui Sousa • Jan 9 '20

Leave it. It helps with the SEO and, if anything, it will show first for beginners when they search for it. It's an important article and the 2020 really doesn't hurt anyone

Ibrahima Ndaw • Jan 10 '20

So, I'll let the title as it is.

Paulo • Jan 9 '20

I personally like includes() for transforming a code like

if (value === 1 || value === 2 || value === 3) {...}

into

if ([1, 2, 3].includes(value)) {...}

Ibrahima Ndaw • Jan 9 '20

It's a very handy method.

Vasyl • Jan 9 '20

Nice article.
But I am confused what is the difference between map and forEach? Output looks the same.

Ibrahima Ndaw • Jan 9 '20

map and forEach technically they do the same job. They both iterate your data holden in a given array. But under the hood they are different. map instead of manipulating the array directly, it will return a new array that contains an image of each element of the array. In the case of forEach, it will manipulate the array directly. That's the difference.

Vasyl • Jan 9 '20

Thanks. Got it now)

Paulo • Jan 9 '20 • Edited

map() takes a function as argument which will run on all elements on the array (getting a transformed value), and then return a new array with all transformed values.
If we run the following array through .map(x => x + 1) we get:

[1, 2, 3]
 |  |  |
 v  v  v
[2, 3, 4]

forEach() will just iterate on the array. Its return value is undefined.

Neither function will mutate the original array.

Ibrahima Ndaw • Jan 9 '20

Great explanations and examples. Thanks again

Pacharapol Withayasakpunt • Jan 10 '20 • Edited

I think everyone know .push(), but recently I need .shift().

I think I might not need double-ended dequeue, yet.

The most important things you should emphasize, IMO, are,

Mutating / Non-mutating
Returns Array / returns something else

Some other should-know, I think, are

Array constructor
Array.from

Ibrahima Ndaw • Jan 10 '20

Great feedback. I will do a post on it later. Thanks again :).

Amin • Jan 9 '20

Hi there, great article thanks!

I think you forgot about another method, which is similar to reduce: reduceRight.

Ibrahima Ndaw • Jan 10 '20

It's a handy method, but it's very similar to reduce except that reduceRight() start reducing the array from the right to the left. And for the case of reduce(), it starts from left to right. Thanks for your comment

Adam Mescher • Jan 13 '20 • Edited

Greatly appreciate your effort for creating this list. It allowed me to get a stronger grasp on the language.

I would like to mention that during my research of these methods you've brought to all of our attention, I found that the flat() method takes an argument for the depth, so while you are correct that myArr.flat() will default to a depth of one, it's also possible to specify larger values, and even include an infinite option.

For example (stolen directly from the MDN web docs):

var arr3 = [1, 2, [3, 4, [5, 6]]];
arr3.flat(2); // [1, 2, 3, 4, 5, 6]

var arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
arr4.flat(Infinity); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Alexandros Dorodoulis • Jan 9 '20

There is one issue with the 2nd example used in the flatmap. The .flat() call at the end isn't needed, if you run the following code you will see that it evaluates to true

JSON.stringify(myAwesomeArray.map(arr => arr * 10)) === JSON.stringify( myAwesomeArray.map(arr => arr * 10).flat())

The issue is that if you execute [1] * 10 it will return 10 and not [10] as you would expect

Ibrahima Ndaw • Jan 10 '20

There is no issue with that code. It's just two examples.
The first is used with flatMap()

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

myAwesomeArray.flatMap(arr => arr * 10)
//-------> Output : [10, 20, 30, 40, 50]

And the second uses flat() and map().

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

// With .map() and .flat()
myAwesomeArray.map(arr => arr * 10).flat()
//-------> Output : [10, 20, 30, 40, 50]

As a side note, with flatMap(), the map() function is applied first and flat() comes after.

Russ • Jan 10 '20

Awesome post, thank you :). Just a curious question (junior dev), should we avoid using forEach() because it'll mutate the data?

Ibrahima Ndaw • Jan 10 '20

forEach() is not bad, it's not like DON'T USE IT. But it depends on the use case.
If you tend to functional programming, immutability and all that kind of stuff, you should use map() more often than forEach().

forEach is preferable when you're planning to do something like logging out your data or save the data to a database. But don't change your data with forEach.
Instead, use map() when you plan to change or alternate your data. You can even use filter(), sort(), reduce() etc. in conjunction with map(). You can still do the same thing with forEach(), but it's preferable to use map() because it manipulates your data in an immutable way.
Some folks also say that map() is faster than forEach() regarding performance.
However, in the end, It depends on what you’re trying to accomplish.

Russ • Jan 10 '20

Awesome, thank you for the detailed response :)

Akash Kava • Jan 10 '20

Last one is wrong, myAwesomeArray.map(arr => arr * 10).flat()

It should be

myAwesomeArray.flat().map(arr => arr * 10)

Ibrahima Ndaw • Jan 10 '20

It's not wrong. I've just followed the way flatMap() work. It applies map() first and flat() after. Therefore for the example of flat() and map() i used the same method. But if you want too you can flat the array first.

Akash Kava • Jan 10 '20

Try it in JSFiddler, Chrome, it does not work, it is wrong !!

Ibrahima Ndaw • Jan 10 '20

I've not tested with your array. You're right. Thanks for your comment.

Michal Novák • Jan 11 '20 • Edited

Sorry, but NOW it's incorrect - it was correct before. That's beacuse the flat part in flatMap() works differently, than .flat(1) alone, it only works for [[1], [2]], but not for [[1, 2]].

[[1, 2], 3, 4].flatMap(x => [x * 2])
// [NaN, 6, 8]

[[1, 2], 3, 4].map(x => [x * 2]).flat(1)
// [NaN, 6, 8]

See? flatMap will put the NaN there as well, try it for yourself.

I'd say it's either a bug in the implementation or on MDN description:

The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. It is identical to a map() followed by a flat() of depth 1, but flatMap() is often quite useful, as merging both into one method is slightly more efficient.

and later:

The flatMap method is identical to a map followed by a call to flat of depth 1.

Example on MDN:

let arr1 = [1, 2, 3, 4];

arr1.map(x => [x * 2]); 
// [[2], [4], [6], [8]]

arr1.flatMap(x => [x * 2]);
// [2, 4, 6, 8]

// only one level is flattened
arr1.flatMap(x => [[x * 2]]);
// [[2], [4], [6], [8]]

Which translates to this one-liner:

[1, 2, 3, 4].map(x => [x * 2]).flat(1);
// [2, 4, 6, 8]

and not the other way around:

[1, 2, 3, 4].flat(1).map(x => [x * 2]);
// [[2], [4], [6], [8]]

Jater Loh • Jan 9 '20

Great article, love that every single one of them is to the point and clear.

One small typo to fix is for Number 13 (Includes), the output should be "true".

Ibrahima Ndaw • Jan 9 '20

Sorry, for the mistyping. Now it's good :). Thanks again for reading it

View full discussion (71 comments)

DEV Community

15 must-know JavaScript array methods in 2020

1. some()

2. reduce()

3. every()

4. map()

5. flat()

6. filter()

7. forEach()

8. findIndex()

9. find()

10. sort()

11. concat()

12. fill()

13. includes()

14. reverse()

15. flatMap()

Top comments (71)

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