How to Pad a Number with Leading Zeros in JavaScript?

How to Pad a Number with Leading Zeros in JavaScript

JavaScript lacks a built-in function for padding numbers with leading zeros. However, you can achieve this padding using various methods.

Solution 1: Using String.prototype.padStart()

For ES2017 and above, you can leverage the String.prototype.padStart() method:

String(n).padStart(4, '0');

This will pad the number n with zeros until it reaches a total length of 4. For example:

n = 9;
String(n).padStart(4, '0'); // '0009'

n = 10;
String(n).padStart(4, '0'); // '0010'

Solution 2: Using Array.prototype.join()

For pre-ES2017 browsers, you can use a combination of Array.prototype.join() and string concatenation:

function pad(n, width, z) {
  z = z || '0';
  n = n + '';
  return n.length >= width ? n : new Array(width - n.length + 1).join(z) + n;
}

This function first checks if the number's string length is already greater than or equal to the desired width. If not, it initializes an array with a length equal to the width minus the number's current length plus 1 and joins it with the zero character. This effectively creates a string of zeros of the appropriate length, which is then concatenated with the number.

Example Usage:

pad(10, 4);      // 0010
pad(9, 4);       // 0009
pad(123, 4);     // 0123

pad(10, 4, '-'); // --10

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