flatten-iter.ts

function flatten<T>(arr: Array<T|Array<T>>): Array<T> {
    let result: Array<T> = [];
    let stack: Array<{ arr: Array<T | Array<T>>, idx: number }> =
        [{ arr, idx: 0 }];
    main: while (stack) {
        let {arr: current, idx} = stack.pop();
        for (; idx < current.length; idx++) {
            if (current[idx] instanceof Array) {
                stack.push({arr: current, idx});
                stack.push({arr: <Array<T>>current[idx], idx: 0});
                continue main;
            } else {
                result.push(<T>current[idx]);
            }
        }
    }
    return result; 
}

console.log(flatten([1, [2, [3, 4], 5], 6, [7]]));