Pad a number with leading zeros in JavaScript [duplicate]

Answer 1

function padToFour(number) {
  if (number<=9999) { number = ("000"+number).slice(-4); }
  return number;
}

Something like that?

Bonus incomprehensible-but-slicker single-line ES6 version:

let padToFour = number => number <= 9999 ? `000${number}`.slice(-4) : number;

ES6isms:

• let is a block scoped variable (as opposed to var’s functional scoping)
• => is an arrow function that among other things replaces function and is prepended by its parameters
• If a arrow function takes a single parameter you can omit the parentheses (hence number =>)
• If an arrow function body has a single line that starts with return you can omit the braces and the return keyword and simply use the expression
• To get the function body down to a single line I cheated and used a ternary expression