flatten.js

export default function flatten(a) {
    let flat = [];
    for(var item of a){
        if(Array.isArray(item)){
            flat = flat.concat(flatten(item));
        }else{
            flat.push(item);
        }
    }
    return flat;
}

// To run this use babel-node with e2015 preset

import flatten from './flatten';

let a = [1, [[2], [3]], [4, [5], 6, [[[7, 8, 9]]]]];
console.log(flatten(a));