convert ‘1’ to ‘0001’ in JavaScript [duplicate]

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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 😉

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