Create an array with same element repeated multiple times

Question

In Python, where [2] is a list, the following code gives this output:

[2] * 5 # Outputs: [2,2,2,2,2]

Does there exist an easy way to do this with an array in JavaScript?

I wrote the following function to do it, but is there something shorter or better?

var repeatelem = function(elem, n){
    // returns an array with element elem repeated n times.
    var arr = [];

    for (var i = 0; i <= n; i++) {
        arr = arr.concat(elem);
    };

    return arr;
};

Answer 1

...and Array.fill() comes to the rescue!

Used to write it all manually before knowing this one 🤦🏽‍♂️

Array(5).fill('🔥')  // =>  ['🔥','🔥','🔥','🔥','🔥']

Array(4).fill(0)  // =>  [0, 0, 0, 0]

You can also easily create a sequential array using fill() + map()

Array(4).fill('').map((_, i) => i + ' 🌵') // =>  ['0 🌵','1 🌵','2 🌵','3 🌵']


Array(3).fill(' 🌻').map((flower, i) => i + flower) // =>  ['0 🌻','1 🌻','2 🌻']

---

❗️Just be careful when creating objects and arrays using .fill() as they are referenced types❗️

That means Javascript will consider all the created items as being the same object (what may introduce unexpected bugs in case you want to further interact with the created objects)

// ❌ Careful when using .fill() with objects and arrays:

Array(3).fill({ value: 2 })  // =>  [{ value: 2 },{ value: 2 },{ value: 2 }]

The above line works, but it would be much safer to stick to the .fill().map() pattern. Like this:

// 👍🏽 Much better!

Array(3).fill().map(item => ({ value: 2 }))

Answer 2

Turn to string and return to array.

['1', '2'] => '1--2' => repeat

['1', '2'].join('--').repeat(2).split('--') will return ['1','2','1','2',]

Answer 3

>>> Array.apply(null, Array(10)).map(function(){return 5})
[5, 5, 5, 5, 5, 5, 5, 5, 5, 5]
>>> //Or in ES6
>>> Array.from({length: 10}, () => 5)
[5, 5, 5, 5, 5, 5, 5, 5, 5, 5]

Answer 4

In ES6 using Array fill() method

console.log(
  Array(5).fill(2)
)
//=> [2, 2, 2, 2, 2]

Answer 5

If you need to repeat an array, use the following.

Array(3).fill(['a','b','c']).flat() 

will return

Array(9) [ "a", "b", "c", "a", "b", "c", "a", "b", "c" ]

Answer 6

You can use the SpreadOpeator and the map() function to create an array with the same element repeated multiple times.

function fillArray(value,len){
       return [...Array(len).keys()].map(x=> value);
   }

Answer 7

This could be another answers.

let cards = ["A","2","3","4","5","6","7","8","9","10","J","Q","K"];

let totalCards = [...cards, ...cards, ...cards, ...cards];

Answer 8

you can try:

Array(6).join('a').split(''); // returns ['a','a','a','a','a'] (5 times)

Update (01/06/2018):

Now you can have a set of characters repeating.

new Array(5).fill('a'); // give the same result as above;
// or
Array.from({ length: 5 }).fill('a')

Note: Check more about fill(...) and from(...) for compatibility and browser support.

Update (05/11/2019):

Another way, without using fill or from, that works for string of any length:

Array.apply(null, Array(3)).map(_ => 'abc') // ['abc', 'abc', 'abc']

Same as above answer. Adding for sake of completeness.

Answer 9

In the Node.js REPL:

> Array.from({length:5}).map(x => 2)
[ 2, 2, 2, 2, 2 ]

Answer 10

Use this function:

function repeatElement(element, count) {
    return Array(count).fill(element)
}

>>> repeatElement('#', 5).join('')
"#####"

Or for a more compact version:

const repeatElement = (element, count) =>
    Array(count).fill(element)

>>> repeatElement('#', 5).join('')
"#####"

Or for a curry-able version:

const repeatElement = element => count =>
    Array(count).fill(element)

>>> repeatElement('#')(5).join('')
"#####"

You can use this function with a list:

const repeatElement = (element, count) =>
    Array(count).fill(element)

>>> ['a', 'b', ...repeatElement('c', 5)]
['a', 'b', 'c', 'c', 'c', 'c', 'c']

Answer 11

var finalAry = [..."2".repeat(5).split("")].map(Number);
console.log(finalAry);

Answer 12

Try This:

"avinash ".repeat(5).trim().split(" ");

Answer 13

Improving on Vivek's answer, this works for strings of any length, to populate an array of length n: Array(n+1).join('[string to be repeated][separator]').split('[separator]').slice(0, n)

Answer 14

Can be used as a one-liner too:

function repeat(arr, len) {
    while (arr.length < len) arr = arr.concat(arr.slice(0, len-arr.length));
    return arr;
}

Answer 15

Array.from({length:5}, i => 1) // [1, 1, 1, 1, 1]

or create array with increasing value

Array.from({length:5}, (e, i)=>i) // [0, 1, 2, 3, 4]

Answer 16

I discovered this today while trying to make a 2D array without using loops. In retrospect, joining a new array is neat; I tried mapping a new array, which doesn't work as map skips empty slots.

"#".repeat(5).split('').map(x => 0)

The "#" char can be any valid single character. The 5 would be a variable for the number of elements you want. The 7 would be the value you want to fill your array with.

The new fill method is better, and when I coded this I didn't know it existed, nor did I know repeat is es6; I'm going to write a blog post about using this trick in tandem with reduce to do cool things.

http://jburger.us.to/2016/07/14/functionally-create-a-2d-array/

Answer 17

If you are using a utlity belt like lodash/underscore you can do it like this :)

let result = _.map(_.times(foo), function() {return bar})

Answer 18

You can do it like this:

function fillArray(value, len) {
  if (len == 0) return [];
  var a = [value];
  while (a.length * 2 <= len) a = a.concat(a);
  if (a.length < len) a = a.concat(a.slice(0, len - a.length));
  return a;
}

It doubles the array in each iteration, so it can create a really large array with few iterations.

---

Note: You can also improve your function a lot by using push instead of concat, as concat will create a new array each iteration. Like this (shown just as an example of how you can work with arrays):

function fillArray(value, len) {
  var arr = [];
  for (var i = 0; i < len; i++) {
    arr.push(value);
  }
  return arr;
}

Answer 19

In lodash it's not so bad:

_.flatten(_.times(5, function () { return [2]; }));
// [2, 2, 2, 2, 2]

EDIT: Even better:

_.times(5, _.constant(2));
// [2, 2, 2, 2, 2]

EDIT: Even better:

_.fill(Array(5), 2);

Answer 20

In case you need to repeat an array several times:

var arrayA = ['a','b','c'];
var repeats = 3;
var arrayB = Array.apply(null, {length: repeats * arrayA.length})
        .map(function(e,i){return arrayA[i % arrayA.length]});
// result: arrayB = ['a','b','c','a','b','c','a','b','c']

inspired by this answer

Answer 21

Another one-liner:

Array.prototype.map.call([]+Array(5+1),function(){ return '2'; })

Answer 22

I had problems with the mentioned methods when I use an array like

var array = ['foo', 'bar', 'foobar'];
var filled = array.fill(7);

//filled should be ['foo', 'bar', 'foobar', 'foo', 'bar', 'foobar', 'foo']

To get this I'm using:

Array.prototype.fill = function(val){
    var l = this.length;
    if(l < val){
        for(var i = val-1-l; i >= 0; i--){
            this[i+l] = this[i % l];
        }
    }
    return this;
};

Answer 23

[c] * n can be written as:

Array(n+1).join(1).split('').map(function(){return c;})

so for [2] * 5

Array(6).join(1).split('').map(function(){return 2;})

Answer 24

You can also extend the functionality of Array like so:

Array.prototype.fill = function(val){
    for (var i = 0; i < this.length; i++){
        this[i] = val;
    }
    return this;
};
// used like:
var arry = new Array(5)​.fill(2);
// or
var arry = new Array(5);
arry.fill(2);


​console.log(arry);​ //[2, 2, 2, 2, 2] 

I should note that extending the functionality of built-in objects can cause problems if you are working with 3rd-party libraries. Always weigh this into your decisions.

Answer 25

This function creates an array of (length) elements where each element equals (value) as long as (value) is an integer or string of an integer. Any decimal numbers will be truncated. If you do want decimal numbers, replace "parseInt(" with "parseFloat("

function fillArray(length, intValue) {
     var vals = (new Array(length + 1)).join(intValue + '|').split('|').slice(0,length);
     for(var i = 0; i < length; i += 1) {
         vals[i] = parseInt(vals[i]);
     }
     return vals;
}

Examples:

fillArray(5, 7) // returns [7,7,7,7,7]
fillArray(5, 7.5) // returns [7,7,7,7,7]
fillArray(5, 200) // returns [200,200,200,200,200]

Answer 26

No easier way. You need to make a loop and push elements into the array.