How can I format an integer to a specific length in javascript?

Question

I have a number in Javascript, that I know is less than 10000 and also non-negative. I want to display it as a four-digit number, with leading zeroes. Is there anything more e

Answer 1

I think the most compact yet intuitive way is:

function toFixedLength(input, length, padding) {
    padding = String(padding || "0");
    return (padding.repeat(length) + input).slice(-length);
}

The slice method here could be replaced with substr if that is more intuitive to the coder.