How to duplicate elements in a js array?

Question

Whats the easiest way (with \"native\" javascript) to duplicate every element in a javascript array?

The order matters.

For example:

Answer 1

0/2  =  0    =  0  |0  =  0
1/2  =  0.5  =  0.5|0  =  0
2/2  =  1    =  1  |0  =  1
3/2  =  1.5  =  1.5|0  =  1
4/2  =  2    =  2  |0  =  2
5/2  =  2.5  =  2.5|0  =  2
6/2  =  3    =  3  |0  =  3
7/2  =  3.5  =  3.5|0  =  3

Treat |0 as Math.floor

---

In code this could look like this:

for (let i = 0; i < a.length * 2; i++) {
  a[i] = a[i / 2 | 0]
}

Because immutability is preferable, one could do something like this:

function repeatItems(a, n) {
  const b = new Array(a.length * n)
  for (let i = 0; i < b.length; i++) {
    b[i] = a[i / n | 0]
  }
  return b
}

Unreadable ES6 spaghetti code:

const repeatItems = (a, n) => Array.from(Array(a.length * n), (_, i) => a[i / n | 0])

Answer 2

Just splice a little bit.

var a = [2, 3, 1, 4],
    i = a.length;
while (i--) {
    a.splice(i, 0, a[i]);
}
document.write('<pre>' + JSON.stringify(a, 0, 4) + '</pre>');

Answer 3

Basically you can use flatMap in ES19

a = [1, 2, 3, 4];
a.flatMap(i => [i,i]); // [1, 1, 2, 2, 3, 3, 4, 4]

Also you can customize the repetitions number like this:

a = [1, 2, 3, 4];
const dublicateItems = (arr, numberOfRepetitions) => 
    arr.flatMap(i => Array.from({ length: numberOfRepetitions }).fill(i));

dublicateItems(a, 3);

Answer 4

I came across this issue in my application, so I created this function:

function duplicateElements(array, times) {
  return array.reduce((res, current) => {
      return res.concat(Array(times).fill(current));
  }, []);
}

To use it, simple pass the array, and the number of times you want the elements to be duplicated:

duplicateElements([2, 3, 1, 4], 2);
// returns: [2, 2, 3, 3, 1, 1, 4, 4]

Answer 5

ES6 way of life (based on axelduch's answer)

const arr = [2, 3, 1, 4].reduce((res, current) => [...res, current, current], []);

console.log(arr);