What would be the equivalent of this without using a for loop?

Unsure what array method to use

```
function func(x){
    var y = [];
    for(i=0;i<x.length;i++){
        y.push(x[i]);
        y.push(x[i]);
    }
    return y;
}

var input = [1,2,3,4,5,6];

console.log(func(input));// returns [1,1,2,2,3,3,4,4,5,5,6,6]

```