Question

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Is there a JavaScript function that can pad a string to get to a determined length?

How can I convert convert ‘1’ to ‘0001’ in JavaScript without using any 3rd party libraries. I have done this in php using spritf: $time = sprintf('%04.0f',$time_arr[$i]);

Answer 1

This is a clever little trick (that I think I’ve seen on SO before):

var str = "" + 1
var pad = "0000"
var ans = pad.substring(0, pad.length - str.length) + str

JavaScript is more forgiving than some languages if the second argument to substring is negative so it will "overflow correctly" (or incorrectly depending on how it’s viewed):

That is, with the above:

• 1 -> "0001"
• 12345 -> "12345"

Supporting negative numbers is left as an exercise 😉