A simple string padding function written in ES5 with Closure compiler annotations

String.pad.js

/**
 * String.pad.js
 *  A simple string padding function written in ES5 with Closure compiler annotations
 */


/**
 * Enum for the padding alignment
 *  @enum {number}
 */
String.padding = {
	LEFT: 0,
	RIGHT: 1,
};


/**
 * Apply padding to a string
 *  @this  {string}             this            The input string to pad
 *  @param {number}             length          Length of the padded string
 *  @param {string}             character       Character to pad the string with
 *  @param {String.padding}     direction       The side of the string to add padding
 */
String.prototype.pad = function(length, character, direction) {
	
	// Get the arguments
	length = parseInt(length, 10);
	character = (character || ' ').toString();
	direction = (direction || String.padding.LEFT);
	
	// Validate the arguments
	if (length < 0) throw new Error('Invalid length');
	if (character.length < 1) throw new Error('Invalid character');
	
	// Create a string we can modify
	var paddedString = this.toString();
	
	// Loop until the padded string matches the length
	while (paddedString.length < length) {
		
		// Get the padding
		var padding = character.substring(0, length - paddedString.length);
		
		// Add the character based on the direction
		paddedString = (direction === String.padding.LEFT) ?
			(padding + paddedString)
		:	(paddedString + padding);
		
	}
	
	// Return the result
	return paddedString;
	
};


/** Create a prototype-less function **/
String.pad = (function() {}).call.bind(String.prototype.pad);


/** The following expression should evaluate to true **/
((2).toString().pad(4, '0') === '0002' &&
(14).toString().pad(4, '20') === '2014' &&
'Hello, '.pad(12, 'World', String.padding.RIGHT) === 'Hello, World' &&
'Hello, '.pad(13, 'World', String.padding.RIGHT) === 'Hello, WorldW' &&
String.pad(19, 4, 90, String.padding.RIGHT) === '1990' &&
String.pad(1 / 0) === 'Infinity');