Flatten Array

flatten.js

var arr = [[1,2,[3,4,[5,6]]],[4,10,23,[3,5,[56]]]];

function flattenArr(val) {
    if(val instanceof Array) {

        return val.reduce(function(acc, val) {
            return acc.concat(flattenArr(val));
        },[]);
    }

    return [val];
}

var newArr = arr.reduce(function(acc, val) {
    return acc.concat(flattenArr(val));
}, []);

console.log(newArr);