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15 must-know JavaScript array methods in 2020

ibrahima92 profile image Ibrahima Ndaw Originally published at ibrahima-ndaw.com Updated on ・4 min read

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.

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]

Discussion

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mdamaceno profile image
Marco Damaceno

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

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idanarye profile image
Idan Arye

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

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dexygen profile image
George Jempty

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

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ibrahima92 profile image
Ibrahima Ndaw Author

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.

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Jochem Stoel

@idan haha that is brilliant response.

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ibrahima92 profile image
Ibrahima Ndaw Author

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.

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ibrahima92 profile image
Ibrahima Ndaw Author

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

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itzami profile image
Rui Sousa

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

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ibrahima92 profile image
Ibrahima Ndaw Author

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

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pszndr profile image
Paulo

I personally like includes() for transforming a code like

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

into

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

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Ibrahima Ndaw Author

It's a very handy method.

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vsment profile image
Vasyl

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

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ibrahima92 profile image
Ibrahima Ndaw Author

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.

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Michael Kohl

The example is not particularly well chosen. map is about data transformation, so using it for a side-effect like console.log leads to confusion.

Something like the following would maybe better illustrate the point:

const myAwesomeArray = [5, 4, 3, 2, 1]
const squares = myAwesomeArray.map(x => x * x)
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ibrahima92 profile image
Ibrahima Ndaw Author

Even with your example, it will be not that clear. I think the difference is made by the definition. But anyway, I will update the code with your example. Thanks

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citizen428 profile image
Michael Kohl

It's at least clearer that the intent of map is returning an array with modified data, not performing an action on each element.

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ibrahima92 profile image
Ibrahima Ndaw Author

I totally agree. I've updated the example with yours. Thanks again for your suggestion.

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vsment profile image
Vasyl

Thanks. Got it now)

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pszndr profile image
Paulo

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.

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Ibrahima Ndaw Author

Great explanations and examples. Thanks again

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aminnairi profile image
Amin

Hi there, great article thanks!

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

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ibrahima92 profile image
Ibrahima Ndaw Author

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

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Pacharapol Withayasakpunt

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
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ibrahima92 profile image
Ibrahima Ndaw Author

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

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Alexandros Dorodoulis

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

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Ibrahima Ndaw Author

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.

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Adam Mescher

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]
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Jater Loh

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".

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ibrahima92 profile image
Ibrahima Ndaw Author

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

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Akash Kava

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

It should be

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

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ibrahima92 profile image
Ibrahima Ndaw Author

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.

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Akash Kava

Error

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

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ibrahima92 profile image
Ibrahima Ndaw Author

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

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mikiqex profile image
Michal Novák

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]]
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Russ

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

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Ibrahima Ndaw Author

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.

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Russ

Awesome, thank you for the detailed response :)

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Michel Renaud

Nice.

The output for 13 (includes) is wrong (copy/paste demon :)).

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ibrahima92 profile image
Ibrahima Ndaw Author

Sorry, It's just a mistyping. Now it's good :)
Thanks again.

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metalmikester profile image
Michel Renaud

If I had $10 for each time something like that happened to me, I could buy my own country! :D

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ibrahima92 profile image
Ibrahima Ndaw Author

:) for sure, we have the same issue.

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Nerando Johnson

Love it, please provide a place where we can go to practice all these skills.

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ibrahima92 profile image
Ibrahima Ndaw Author

I'm glad you like it. Maybe I'll create a github repo for that.

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Nerando Johnson

Appreciate it or you can just edit it and point a known resource to practice each.

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Costin Manda

Some note towards C# people who look at this and think it's the same as LINQ. It is not, as most of these methods create a new array as a result. Usually it doesn't matter as much, but consider what happens if the array is very large and you first map it to some complex and heavy calculation and only then you filter it. It may seem obvious, but only if you actually think about it, which to my shame I only did recently.

Also, shameless plus for my own library that uses ES6 iterators/generators to emulate LINQ in Javascript: siderite.dev/blog/linq-in-javascri... ;)

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Enrique

Thanks for this post , it was very useful.

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Ibrahima Ndaw Author

I'm glad you like it. Thanks for your comment

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Jochem Stoel

I consider myself a pretty awesome JavaScript developer and often express that in my interaction with others but I honestly had never heard of Array.some().

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Ibrahima Ndaw Author

It's a very handy method. It happens to everyone, we can't know everything :)

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Prafulla Raichurkar

Good article, thank you!

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Ibrahima Ndaw Author

Thanks you too for reading it.

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Marcos dos Santos Carvalho

Thanks, your post is awesome.

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Calvin

Lovely

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Ibrahima Ndaw Author

Thanks for reading it.

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Freddie Runnding

Love this! I needed it :D

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Ibrahima Ndaw Author

I'm glad it helps, and thanks again for reading it.

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Kim Sean Pusod

I absolutely love this post. Thank you ❤️

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Ibrahima Ndaw Author

I'm glad you like it. Thanks for reading it

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Gustavo

The only difference between some() and includes() is the parameter (the former is a function, the latter an element) or Did I miss something?

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Ibrahima Ndaw Author

Yes, they are kinda similar. They both check if a given element exists in the array or not. They both return true or false as output.

includes() takes the element to check and optionally the starting index.

some() takes a function as a parameter.

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Pablo Hernandez

Thanks for the information

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Ibrahima Ndaw Author

Thanks for reading it

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shroomlife 🍄

great! thank you.

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baloids profile image
BaloiDS

Very useful article, thanks!

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Ibrahima Ndaw Author

I'm glad it helps. Thanks for reading it

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Marcos Aguilera Ely

Thanks a lot, this is one most important post for me, I don't shame it.

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cherucole profile image
Cherucole

I enjoyed every bit of this, simple clear examples. Now depends on how we implement them

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Ibrahim Abdullahi Aliyu

Awesome, namesake. I was discussing with a Senior today about HOF and I just see this.
However I'm confused, what's the difference between filter() and find()

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Edward Iga Kigongo

Hi Ibrahim, thanks for the article. One thing to note is that the implementation for sort is correct but the comments are wrong.

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Ibrahima Ndaw Author

Thanks for reading it. Now it's correct