My javascript utility functions.

utils.js

/**
 * Set the character at a particular index in the given String.
 * 
 * @param str
 *   The string to tweak.
 * @param index
 *   The index where the replacement will take effect.
 * @param chr
 *   The character to set in the given index.
 */
function setCharAt(str,index,chr) {
    if(index > str.length-1) return str;
    return str.substr(0,index) + chr + str.substr(index+1);
}