var arr = [1,2,3,4,[5,5,[1,1,1],6,7],[1,2,[2,3,[100,[23,45,11,[600,599]],101,99],4],3]];

function flattenArray(array) {
    return array.reduce(function(accumulator, current) {
        if (Array.isArray(current)) {
            return accumulator.concat(flattenReduce(current));
        } else {
            return accumulator.concat(current);
        }
    }, []);
}

flattenArray(arr);