How Can I Pad Strings to a Desired Length in JavaScript?

Padding a String in JavaScript

Introduction

JavaScript provides various methods to achieve specific string manipulation tasks. One such task is padding a string to a desired length, filling the empty spaces with characters. This article discusses different ways to pad a string in JavaScript.

Solution Using String.padStart

ECMAScript 2017 (ES8) introduced the String.padStart method for this purpose. It takes two parameters: the desired length and an optional padding string (which defaults to a space).

"Jonas".padStart(10); // Default pad string is a space
"42".padStart(6, "0"); // Pad with "0"
"*".padStart(8, "-/|\"); // produces '-/|\-/|*'

Polyfill for String.padStart

If String.padStart is not supported in your JavaScript environment, you can add it as a polyfill:

if (!String.prototype.padStart) {
  String.prototype.padStart = function(length, padString) {
    padString = padString || " ";
    if (this.length >= length) return this;
    else return padString.repeat(length - this.length) + this;
  };
}

Using String Concatenation

A simple way to pad a string is to concatenate it with the desired padding characters.

var n = 123

String("00000" + n).slice(-5); // returns 00123
("00000" + n).slice(-5); // returns 00123
("     " + n).slice(-5); // returns "  123" (with two spaces)

Extending the String Object

You can also extend the String object to include a paddingLeft method for your convenience.

String.prototype.paddingLeft = function (paddingValue) {
   return String(paddingValue + this).slice(-paddingValue.length);
};

Example Usage

function getFormattedTime(date) {
  var hours = date.getHours();
  var minutes = date.getMinutes();

  hours = hours.toString().paddingLeft("00");
  minutes = minutes.toString().paddingLeft("00");

  return "{0}:{1}".format(hours, minutes);
};

String.prototype.format = function () {
    var args = arguments;
    return this.replace(/{(\d+)}/g, function (match, number) {
        return typeof args[number] != 'undefined' ? args[number] : match;
    });
};

This code will return a time in the format "15:30".

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