Javascript tips and tricks I've found useful. Contributions welcome (just visit the source, make sure you are logged in and hit edit - or fork and patch).

Here beginneth the tips

Deep Copy

Warning: full deep copy is hard. The following are only appropriate for "simple" objects.

JQuery

Copy (clone) in javascript with JQuery::

// Shallow copy
var newObject = jQuery.extend({}, oldObject);

// Deep copy
var newObject = jQuery.extend(true, {}, oldObject);

Underscore

Note underscore does not do a deep copy at all -- it will only do copy on first level attributes::

_.extend({}, myobject);

JSON

Deserialize to and from JSON (warning: not at all performant!)::

var newobj = JSON.parse(JSON.stringify(oldobj));

Check if a variable is undefined

if (typeof variable === undefined) {
  // do something
}

Simile Inheritance in Javascript

John Resig's original

From http://ejohn.org/blog/simple-javascript-inheritance/

/* Simple JavaScript Inheritance
 * By John Resig http://ejohn.org/
 * MIT Licensed.
 */
// Inspired by base2 and Prototype
(function(){
  var initializing = false, fnTest = /xyz/.test(function(){xyz;}) ? /\b_super\b/ : /.*/;
  // The base Class implementation (does nothing)
  this.Class = function(){};
  
  // Create a new Class that inherits from this class
  Class.extend = function(prop) {
    var _super = this.prototype;
    
    // Instantiate a base class (but only create the instance,
    // don't run the init constructor)
    initializing = true;
    var prototype = new this();
    initializing = false;
    
    // Copy the properties over onto the new prototype
    for (var name in prop) {
      // Check if we're overwriting an existing function
      prototype[name] = typeof prop[name] == "function" && 
        typeof _super[name] == "function" && fnTest.test(prop[name]) ?
        (function(name, fn){
          return function() {
            var tmp = this._super;
            
            // Add a new ._super() method that is the same method
            // but on the super-class
            this._super = _super[name];
            
            // The method only need to be bound temporarily, so we
            // remove it when we're done executing
            var ret = fn.apply(this, arguments);        
            this._super = tmp;
            
            return ret;
          };
        })(name, prop[name]) :
        prop[name];
    }
    
    // The dummy class constructor
    function Class() {
      // All construction is actually done in the init method
      if ( !initializing && this.init )
        this.init.apply(this, arguments);
    }
    
    // Populate our constructed prototype object
    Class.prototype = prototype;
    
    // Enforce the constructor to be what we expect
    Class.prototype.constructor = Class;

    // And make this class extendable
    Class.extend = arguments.callee;
    
    return Class;
  };
})();

Usage::

var Person = Class.extend({
  init: function(isDancing){
    this.dancing = isDancing;
  }
});
var Ninja = Person.extend({
  init: function(){
    this._super( false );
  }
});

var p = new Person(true);
p.dancing; // => true

var n = new Ninja();
n.dancing; // => false 

Steffen Rusitchka version

/* Steffen Rusitchka version
 * <http://www.ruzee.com/blog/2008/12/javascript-inheritance-via-prototypes-and-closures>
 * Creative Commons Attribution licensed
 */
(function(){
  var isFn = function(fn) { return typeof fn == "function"; };
  PClass = function(){};
  PClass.create = function(proto) {
    var k = function(magic) { // call init only if there's no magic cookie
      if (magic != isFn && isFn(this.init)) this.init.apply(this, arguments);
    };
    k.prototype = new this(isFn); // use our private method as magic cookie
    for (key in proto) (function(fn, sfn){ // create a closure
      k.prototype[key] = !isFn(fn) || !isFn(sfn) ? fn : // add _super method
        function() { this._super = sfn; return fn.apply(this, arguments); };
    })(proto[key], k.prototype[key]);
    k.prototype.constructor = k;
    k.extend = this.extend || this.create;
    return k;
  };
})();

Usage::

var X = PClass.create({
  val: 1,
  init: function(cv) { 
    this.cv = cv; 
  },
  test: function() { 
    return [this.val, this.cv].join(", "); 
  }
});

var Y = X.extend({
  val: 2,
  init: function(cv, cv2) { 
    this._super(cv); this.cv2 = cv2; 
  },
  test: function() { 
    return [this._super(), this.cv2].join(", "); 
  }
});

var x = new X(123);
var y = new Y(234, 567);