javascript - 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:

a = [2, 3, 1, 4]
// do something with a
a
// a is now [2, 2, 3, 3, 1, 1, 4, 4]

Answer 1

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]);
}, []);