How Can I Pad a Number with Leading Zeros in JavaScript?

Padding a Number with Leading Zeros in JavaScript

When dealing with JavaScript, the need often arises to pad a number with leading zeros. For instance, the number 9 becomes "0009," and 10 becomes "0010," always adhering to a four-digit format.

Plain-Jane Method

A straightforward approach involves subtracting the number from 4 to determine the necessary number of zeros. While functional, it lacks flair.

ES2017's padStart() Method

In ES2017, the String.prototype.padStart() method offers a more elegant solution. It appends the specified character (by default, a space) to the left of the string until it reaches the desired length.

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

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

Generic Padding Function

In scenarios where padStart() may not be an option, a generic padding function can be utilized:

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 employs an array's ability to accept a number as an argument and create an array with that length, populated with undefined elements. When joining these elements with a zero (or other specified character), a string is generated with the appropriate number of leading zeros.

Example Usage

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

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

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