Array Flattening (Iterative Version)

flatten.js

function flatten(xs) {
    var ys = [];
    var stack = [[0, xs]];  

    while(stack.length > 0) {
        var [index, arr] = stack.pop();
	    
        while(index < arr.length) {
            if (Array.isArray(arr[index])) {
                stack.push([index+1, arr]);
                stack.push([0, arr[index]]);
                break;  
            } else { 
                ys.push(arr[index]); 
            }
            index++;
        }
    }
    return ys;
}

flatten([1, [2, [3, [4, [5, [6, [7, [8, [9, [10]]]]]]]]]])
flatten([1, [2, [3, [4, [5, [6, [7, [8, [9, [10], 11], 12], 13], 14], 15], 16], 17], 18], 19])