Flatten a multidimensional array

function stillContainsArray(arr) {
    return arr.some(item => item instanceof Array);
}

function concat(arr) {
    return arr.reduce((a,b) => a.concat(b), []);
}

function flatten(arr) {
    while(stillContainsArray(arr)) {
        arr = concat(arr);
    }
    return arr;
}

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]];
flatten(arr).sort((a,b) => a - b);