flatten an array of arbitrarily nested arrays of integers into a flat array of integers e.g. [[1,2,[3]],4] -> [1,2,3,4].

flatten-array.js

function flattenArray(arr){
    return arr.reduce(function(initial,item){
        if(typeof item=="number"){
             initial.push(item);
        }else{
            initial=initial.concat(flattenArray(item))
        }
        return initial;
    },[])
}