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

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]);
}, []);