Filling Array with Alphanumeric values
Unless you confirm the speed is a problem in the code profiler, there's no need to optimize it.

And if you want to make js code run fast, just avoid using one of the most expensive operations: invocation of a callback/function (you had 6 of them), maximize the workload inside the engine by using existing native functions and minimizing amount of time spent in interpreted code.

In this case switching to string manipulation makes the code 2.5 times faster as measured here on 10k repetitions.

let getNums = length =&gt; {
    var letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    var result = letters.substr(0, length).split('');
    for (var i = 1, numberedCount = Math.floor((length - 1)/26); i &lt;= numberedCount; i++) {
        length -= letters.length;
        result.push.apply(result,
            letters.substr(0, length).replace(/./g, '$&amp;' + i + ' ').trim().split(' '));
    }
    return result;
}