Terse way to intersperse element between all elements in JavaScript array?

Question

Say I have an array var arr = [1, 2, 3], and I want to separate each element by an element eg. var sep = \"&\", so the output is [1, \"&a

Answer 1

you can also use the following:

var arr =['a', 'b', 'c', 'd'];
arr.forEach(function(element, index, array){
    array.splice(2*index+1, 0, '&');
});
arr.pop();