Repeat an array with multiple elements multiple times in JavaScript

Question

In JavaScript, how can I repeat an array which contains multiple elements, in a concise manner?

In Ruby, you could do

irb(main):001:0> ["a", "b", "c"] * 3 => ["a", "b", "c", "a", "b", "c", "a", "b", "c"] 

I looked up the lodash library, and didn't find anything that was directly applicable. Feature request: repeat arrays. is a feature request for adding it to lodash, and the best workaround given there is

const arrayToRepeat = [1, 2, 3]; const numberOfRepeats = 3; const repeatedArray = _.flatten(_.times(numberOfRepeats, _.constant(arrayToRepeat))); 

The questions Most efficient way to create a zero filled JavaScript array? and Create an array with same element repeated multiple times focus on repeating just a single element multiple times, whereas I want to repeat an array which has multiple elements.

Using reasonably well-maintained libraries is acceptable.

Answer 1

No need for any library, you can use Array.from to create an array of arrays you want repeated, and then flatten using [].concat and spread:

const makeRepeated = (arr, repeats) =>    [].concat(...Array.from({ length: repeats }, () => arr));      console.log(makeRepeated([1, 2, 3], 2));

On newer browsers, you can use Array.prototype.flat instead of [].concat(...:

const makeRepeated = (arr, repeats) =>    Array.from({ length: repeats }, () => arr).flat();      console.log(makeRepeated([1, 2, 3], 2));

Answer 2

You can use the Array constructor along with its fill method to fill it a number of times of the array you want to repeat, then concat them (the subarrays) into a single array:

const repeatedArray = [].concat(...Array(num).fill(arr)); 

Note: On older browsers (pre-ES6), you can use Function#apply to mimic the rest syntax above (concat will be called with each of the sub arrays passed to it as argument):

var repeatedArray = [].concat.apply([], Array(num).fill(arr)); 

Example:

const arrayToRepeat = [1, 2, 3];  const numberOfRepeats = 3;    const repeatedArray = [].concat(...Array(numberOfRepeats).fill(arrayToRepeat));    console.log(repeatedArray);