string functions for js

string.js

/**
 * number of occurences of substring
 */
String.prototype.occurences = function(str, from, to) {
  return this.substr(from||0, (to||this.length)-(from||0)).split(str).length-1;
};

/**
 * whether string starts with another string
 */
String.prototype.startsWith = function(str) {
  if(typeof str !== "string")
    return false;

  for(var i in str)
    if(!this[i] || this[i] !== str[i])
      return false;
  
  return true;
}