How to duplicate elements in a js array?

I came up with something similar to tymeJV's answer

[2, 3, 1, 4].reduce(function (res, current, index, array) {
    return res.concat([current, current]);
}, []);

I suppose you could do:

var duplicated = a.map(function(item) {
    return [item, item];
}).reduce(function(a, b) { return a.concat(b) });

//duplicated: [2, 2, 3, 3, 1, 1, 4, 4]

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);

My reply to the comment below by @user120242 why so complicated? a.flatMap(i => Array(n).fill(i)) find it here

Basically:

a = [2, 3, 1, 4];
b=[];

for(var i = 0; i< a.length;++i){
  b.push(a[i]);
  b.push(a[i]);
}

a=b;