flatten an array es5

flatten.js

function flatten(a) {
    return a.map(function(val) {
        return (Array.isArray(val)) ? flatten(val) : val;
    });
}

//example usage
var arr = [1, 2, [3, 4], 5, [6, [7, [8]], 9], 10];
var flat = flatten(arr); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]