Pad string

pad-string.js

/**
 * Pads a string so it will reach the requested number of characters. The
 * padding will be added to the left of the original string.
 *
 * @param {string} value   The original string, unpadded.
 * @param {Number} length  The desired length for the value string. Please
 *                         note that this value should be equal to or exceed
 *                         the unpadded length of the value parameter or
 *                         else the result will be a clipped string.
 * @param {string} padChar The character to use to pad the string. When no
 *                         char is provided it will use 0 as the padding
 *                         character.
 *
 * @return {string}        The result is the original string padded on the
 *                         left with as many times the padded char to have
 *                         the resulting string reach the requested width.
 */
function padString(value, length, padChar) {
	if (padChar == null || padChar === '') {
		padChar = '0';
	}

	// To create a string with the specified padChar repeated as many times
	// as the requested length we will create an Array with the requested
	// string length plus 1 extra. We need to extra element because in order
	// to create the string we will join these empty array items together
	// using the padding char as the separator. If the requested length is
	// 2 and we would join these together the separator would appear only
	// once, this is why we need to extra empty array item.
	// When we have a string with the desired length and filled with the
	// padding char we will append the value to the string. Now we take the
	// number of requested chars, starting at the back of the string using
	// substr with a negative number. This way we are sure it will include
	// the value passed along as well as the necessary number of padding
	// characters to reach the desired string length
	return (new Array(length + 1).join(padChar) + value).substr(-length);
}