Extends an object with values from another object. Existing properties are ignored. Useful for ensuring options have default values.

Object.extend.js

Object.prototype.extend = function(obj){
  if(typeof obj !== 'object') return this;
  var current, prop, 
    i = 0, 
    length = arguments.length;
  
  for(; i<length; i++){
    current = arguments[i];
    if(typeof current === 'object'){
      for(prop in current){
        if(!this[prop]){
          this[prop] = current[prop];
        }
      }
    }else{
      console.warn("Input variables should be objects, ignoring.");
    }
  }
  return this;
};

Example:

/*
userOptions = {
  width : 500
}
*/
function doSomething(userOptions){
  var options = {
    height : 100,
    width : 100
  };

  options = userOptions.extend(options);
  /*
    options = { 
      height : 100,
      width : 500
    }
  */
}