Secrets Of The JavaScript Ninja

What Is This Book? Who Is It For?

Secrets Of The JavaScript Ninja is designed for intermediate to advanced JavaScript programmers. It expects you to have a firm understanding of JavaScript and a working knowledge of HTML/CSS.
The strength of this book lies in its ability to force you to revisit concepts most JS programmers (including myself) always take for granted. It puts the focus on functions as objects, function invoking, closures, cross-browsers strategies, etc.

Resig sets the tone pretty early on. After a short introduction in a chapter titled “Arming With Testing And Debugging” he tells us about the importance of testing and goes on to explain how one can create a simple JavaScript assert method and its associated test runner. Testing is usually briefly mentioned towards the end of a textbook. Not this time. Testing is mentioned throughout the book.

So why am I writing about reading “Secrets of the JavaScript Ninja”? Several reasons:

  1. Writing about something helps you remember it better
  2. I’m adding some reference and extra information here and there.
  3. Hopefully this is useful to you, the reader, to a) get a sense of what’s in this book and b) get better at JavaScript!

The remainder of this article is organized in sections, matching roughly the order of the book’s chapters. In each of those sections I go through what picked my interest while reading the book, with a focus on concepts that “clicked” and interesting discoveries.


Created via the Function constructor, functions are special objects with one superpower: they can be invoked. Besides that, functions are object and nothing but objects:

Function Declaration

Two main ways to declare a function:

    // option 1
    function blah() { console.log('called'); }
    // option 2
    var blah = function() { console.log('called'); };

In both cases the function can be called via blah() after its declaration. However there’s a crucial difference: in the first case the function is named blah which means the function will be assigned a property name with the value blah.
In the second case, we’re assigning an anonymous function to the variable blah. The function’s name property in this case will be ''.

Arguments and Invocation

this and arguments are implicit parameters passed to every function. They are just that. Parameters. The difference between this, arguments and standard function parameters is that they are implicit. You won’t see them in function signatures but they’ll be available from within function bodies.


arguments represent a list of all arguments passed to a function. It has array-like accessors (arguments[i]) and properties (arguments.length) but is not an array (arguments.slice(1) fails for instance).
How can you get around that?

Invoking a Function, and this

this is defined as the function context. Available within a function body, this should really be referred to as invocation context, because its value varies based on the way a function is invoked:

Referencing Functions

Referencing functions from within themselves is necessary to achieve recursion. Resig enumerates four ways.

First option: through a named function. Nothing out of the ordinary.

    function fact(x) {
        if (x !== 1) {
            return x * fact(x - 1);
        } else {
            return 1;

Second option: through a method name. Note the danger: if we decide to change that method’s name to newfact, we’ll have to remember to change the inner reference to this.newfact.

    var math = {
        'fact': function (x) {
            if (x !== 1) {
                return x * this.fact(x - 1);
            } else {
                return 1;

Third option: through inline name. This addresses the shortcoming of the previous solution.

    var math = {
        'fact': function myfact(x) {
            if (x !== 1) {
                return x * myfact(x - 1);
            } else {
                return 1;

Last option: callee (going to go away in later versions of JS, use sparingly).

    var math = {
        'fact': function (x) {
            if (x !== 1) {
                return x * arguments.callee(x - 1);
            } else {
                return 1;

Auto-Memoizing Functions

Memoizing by using the object nature of functions:

    function getElements(name) {
        if (!getElements.cache) { getElements.cache = {}; }
        return getElements.cache[name] =
            getElements.cache[name] ||

You can easily guess how useful these techniques are in a library like jQuery (actually probably more in Sizzle, jQuery’s DOM access library).

Function Overloading

Functions have a length property, which corresponds to the number of arguments declared in their signature. By comparing fn.length and arguments.length, function overloading can be implemented in a cool way:

    function addMethod(object, name, fn) {
        var old = object[name];
        object[name] = function() {
            if (fn.length == arguments.length) {
                return fn.apply(this, arguments);
            } else if (typeof old === 'function') {
            return old.apply(this, arguments);

    // Overloading then becomes a piece of cake. Pretty neat right?
    var obj = {};
    addMethod(obj, 'method', function() { /* do something */ });
    addMethod(obj, 'method', function(a) { /* do something with a */ });
    addMethod(obj, 'method', function(a, b) { /* do something with a and b */ });


Closures are probably one of the most confusing concepts when someone with a classical object-oriented background comes to JavaScript.

Resig’s definition of it: “closures allow a function to access all the variables, as well as other functions, that are in scope when the function itself is declared."

Later in the chapter: “That’s what closures are all about. They create a ‘safety bubble,’ if you will, of the function and the variables that are in scope at the point of the function’s declaration […] This ‘bubble,’ containing the function and its variables, stays around as long as the function itself does."

Let’s look at some code samples to go over the main use cases for closures.

Private Variables

    function Counter() {
        var count = 0; // visibility limited to Counter's inner scope
        this.increment = function() { count++; }
        this.getCount = function() { return count; }
    // Can't view/modify count from here. You have to go through Counter.getCount()

Callbacks And Timers

Callback and timers are in the same vain because the idea here is to use a closure to host values that the function scheduled to be called back is going to need when executing. Understanding that a closure’s environment (function, variables) stays around even when the outer scope has finished its execution is fundamental.

     * Callback
    var elem = document.getElementById('my-elem');
        url: '/my-endpoint',
        success: function() {
            // Here we have access to elem because our outer anonymous
            // function had access to it when it was declared.
            elem.innerHTML('callback executed successfully');

     * Timer
     function count() {
        var count = 0;
        setInterval(function() {
            // Here we have access to count for the same reason: our outer
            // anonymous function has access to it when it is declared.
            console.log('Count is ' + count);
        }, 1000);

Binding Function Contexts

Here closures are used to set the context of functions properly.

    // Simple example
    function bind(context, name) {
        return function() {
            // Access to context and name via closure
            return context[name].apply(context, arguments);

    // Example from PrototypeJS
    Function.prototype.bind = function() {
        var fn = this;
        // Turn arguments into a real array
        var args =;
        var ctx = args.shift(); // args is now args[1:n] and object refers
                                // to the first argument
        return function() {
            return fn.apply(
                // `args` refers to the arguments specified at "bind-time"
                // and `arguments` refers to the arguments specified at
                // "call-time".
                // This lets us specify "default" arguments and bind function
                // context at the same time.

    // Sample usage
    var nameSpace = { a: 1, plop: function(b, c) { return this.a + b + c; } }
    var basicBound = basicBind(nameSpace, "plop");
    var bound = nameSpace.plop.bind(nameSpace, 2);
    basicBound(3, 5) // returns 9 (1 + 3 + 5)
    bound(3); // returns 6 (1 + 2 + 3)

Partial Function Application, Currying

Currying and partials are super simple if you understand bind’s definition above. It’s essentially the same trick.

    Function.prototype.curry = function() {
        var fn = this;
        var args =; // "curry-time" args

        return function() {
            return fn.apply(
                // unmodified context
                // concatenation of "curry-time" and "call-time" args, in this order

    Function.prototype.partial = function() {
        var fn = this;
        var args =;

        return function() {
            // Here the args manipulation is a bit more tricky since you can
            // leave an undefined param for partial application later on.
            // (see sample usage)
            var arg = 0;
            for (var i=0; i < args.length && arg < arguments.length; i++) {
                if (args[i] === undefined) {
                    args[i] = arguments[arg];
            return fn.apply(this, args);

    // Sample usage
    var myFunction = function(a, b, c) { return a + b + c; };
    var curried = myFunction.curry(1, 2);
    var partial = myFunction.partial(undefined, 2);
    curried(3); // 6
    partial(1, 4); // 7

Function Wrapping

    function wrap(object, method, wrapper) {
        // Keeps a reference to the original method
        var fn = object[method];

        return object[method] = function() {
            // The wrapper is called with
            // - the original function to be wrapped, as a first argument
            // - the list of arguments to call the original function with
            return wrapper.apply(this, [fn.bind(this)].concat(

    // Sample usage: automatically adds try/catch on event listeners
    wrap(HTMLElement.prototype, "addEventListener", function(old, type, handler, prop) {
        old(type, function() {
            try {
                handler.apply(this, arguments);
            } catch(e) {
                // Handle error
        }, prop);

Immediate Functions

Closures are also crucial in IIFEs (Immediately Invoked Function Expression). Have you always wonder why (function() {})() works the way it does? Well:

    // an IIFE like...
    (function() {/* body */})()
    // EXACTLY the same as...
    var myFunc = function() {/* body */};

This is clever because it means the body of our IIFE is going to:

Hence IIFEs are a nice way to create isolated scopes for independent pieces of functionality. It guarantees no conflict with and no leakage to the outside world. That’s why most libraries, including jQuery, are wrapped in a IIFE.

Note that you can pass params to an IIFE:
(function(a, b) {return a + b;})(1, 2)


JavaScript Prototypes represent a hard-to-grasp concept because most of us think about it with the classical object oriented background in the back of our minds.

In a few sentences Resig made me realize how wrong I’ve been thinking about prototypes: “prototypes are a convenient way to define types of objets, but they’re actually a feature of functions”.

And later: “each object in JavaScript has an implicit property named constructor that references the constructor [function] that was used to create the object. And because the prototype is a property of the constructor [function], each object has a way to find its prototype.
Note that I appended [function] to make you realize that constructors are not a special feature of the language. In fact, a “constructor” is just a function invoked in a given way (using new).

Remember what we discussed before:

References in objects are resolved on the object itself first. However when a property fails to be found on the object itself, a property lookup is made on the object contructor’s prototype property. And you guessed already: if an object constructor’s prototype references an object, lookup can be performed on that object constructor’s prototype. And so on and so forth. That’s what prototype chain lookup means.

Take the following snippet:

    function A() { = 'bar'; }
    var a = new A();
    A.prototype.baz = 'blah';

    console.log(a.baz); // logs 'blah'

What happens in this snippet? A lot!

Type Introspection

There are 3 ways to instrospect into an object’s “type”:

Achieving Inheritance With Prototypes

Achieving inheritance in JavaScript is as simple as using the prototype chain lookup to our advantage so that “inherited” properties will be resolved correctly, thus extending an object’s capabilities.

    function B() {
        this.dataB = function() {...};
    B.prototype.functionB = function() {...};

    function A() {
        this.functionA = function() {...};
    A.prototype = new B();

    var a = new A();

    a.functionA(); // ok
    a.functionB(); // ok!
    a instanceof A; // ok
    a instanceof B; // ok!

Let’s go through what happens when we call a.functionB():

Here’s the complete reference chain:
a->(new A())->constructor->A->prototype
->(new B())->constructor->B->prototype->functionB

Gotchas And Tricks

Prototypes come with lots of bold warnings, the first of which is: be extra careful when modifying/extending native objects' prototypes!
There is only one Array.prototype for a whole page. Be aware that modifying it is similar to modifying a global variable. Actually it’s probably worse because a lot of places are using Array.prototype in non-explicit ways.

In most browsers you can access native DOM element prototype and play with it:

    HTMLElement.prototype.addEventListener = function(type, fn, useProp) {
        console.log('trooooooll!'); // don't do that, seriously!

Second gotcha: use hasOwnProperty to loop through object’s properties.
Otherwise you’ll loop through non-instance properties: = 42;
    var obj = {a: 1, b: 2};

    for (var i in obj) { console.log(i); } 
    //-> logs 'a', 'b' *and 'foo'*

    for (var i in obj) { if (obj.hasOwnProperty(i)) { console.log(i); } }
    //-> logs 'a' and 'b'

Trick to ensure a function is always invoked as a constructor:

    function A() {
        if (!(this instanceof arguments.callee)) {
            return new A();
        } = 'bar';


JavaScript is single-threaded. That’s really important to understand. Once you grasp the fact that the browser has no choice but to queue handlers when events are firing at the same time, understanding why timers cannot be reliable is easy.

Browser APIs are well known: window.setTimeout and window.setInterval to create timers, window.clearTimeout and window.clearInterval to clear them.

Nifty trick for all browsers and IE > 9: timer functions can take arguments! Like so:
window.setInterval(myfunction, 100, arg1, arg2)

Important note! There’s a difference between:

    # Executes `fn` every 10ms by calling setInterval once
    window.setInterval(fn, 10);


    # Executes `fn` every 10ms by calling setTimeout over and over
    window.setTimeout(function fn() {
        window.setTimeout(fn, 10)
    }, 10);

Second version is guaranteed to run every 10ms or more.
First version will try to execute every 10ms regardless of what happened before.

Cutting expensive computation into manageable chunks is an interesting application of timers. Instead of doing a 100% of the work in a big chunk you can choose to schedule manageable chunk via setTimeout thus enabling the browser to do some work after each chunk finishes and before the next one begins. Say a click event happens during a computation: the browser would get a chance to execute the associated handler right after the current chunk is done as opposed to waiting until the end of the whole computation.

Timers are also super useful to build asynchronous test suites and centralized timers.
The idea is to have a single timer handling a queue (of tests to run or of animations/functions to execute) so that the browser is not overwhelmed by multiple timers. This centralized timer technique guarantees order of execution. That’s a perk we don’t get when we use multiple native timers.

Cross Browser Strategies

Authoring cross-browser JavaScript is hard. Resig names 5 major concerns:

Some strategies/advices to deal with browser bugs/differences:

Interestingly enough Resig gives a list of “untestable” browser issues:

DOM Attributes and Properties

There is an incredible amount of quirks involved in getting/setting DOM attributes and properties. Ways to get properties/attributes of DOM nodes:

Note that attribute and property names aren’t always the same. For instance class is a valid attribute name but the associated property is className The other ones are for, readonly, maxlength, cellspacing, rowspan, colspan, tabindex which match respectively to the property names htmlFor, readOnly, maxLength, cellSpacing, rowSpan, colSpan, tabIndex.)

Major quirks pointed out:

Understanding Event Propagation

The order in which events are triggered throughout the DOM tree is not consistent across browsers.

Netscape implements event propagation “outside-in”. An event propagates from the root of the DOM tree all the way through its target (a button on which you clicked for instance). This event propagation model is referred to as capturing.

IE implements it in the exact opposite way: “inside-out”. An event propagates from the target element up to the DOM tree’s root. This event propagation model is referred to as bubbling

When the W3C had to choose, they didn’t. Instead W3C’s model lets you register handlers for capturing or bubbling phase.
Concretely, compliant browsers will do the following when an event is triggered:

  1. start at DOM tree’s root node. See if there are handlers set for capturing phase for our event type. If yes, call them.
  2. go down one level in the tree, do the same (check and call potential handlers set for capturing)
  3. go down one more level, do the same
  4. …etc… (do the same and go down until we reach our event’s target)
  5. on the event’s target (i.e.,the button we clicked) check and call potential handlers set for capturing
  6. on the event’s target, see if there are handlers set for bubbling phase for out event type. If yes, call them.
  7. go up one level in the tree, do the same (check and call potential handlers set for bubbling)
  8. go up one more level, do the same
  9. …etc… (do the same and go up until we reach the DOM tree’s root)
  10. on DOM tree’s root node, check and call potential handlers set for bubbling

The standard API elem.addEventListener(type, handler, useCapture) has 3 params. First one is the type of event to listen to (‘click’, ‘focus’, etc), second is the handler we want to trigger when the type of event we’re listening to happens. The third parameter useCapture is the propagation phase we’re interested in.

Another important thing to consider: browsers have hooks to stop event propagation at any point. Inside a handler, e.stopPropagation() and e.cancelBubble = true; (in IE) will do just that.

That was a lengthy explanation but I think it describes what happens fairly accurately. I hope that it will help you understand a few things. Namely:

A good page to help you understand that if the explanation above didn’t stick: Quirksmode On Events

jQuery’s Event System

At its core jQuery’s event system doesn’t rely much on the browser to work. There are so many quirks that jQuery had to came up with a solution to implement event binding, unbinding and triggering in a consistent manner.
Key ideas:

This is really clever because it enables inventory of handlers at any point in time and programmatic triggering of registered handler.

If you’re interested in the nitty-gritty details head to jQuery’s source

Concept Of Event Delegation

jQuery’s .delegate(type, selector, handler) (or .on(type, selector, handler) in later versions) seems really magic if you don’t understand the concept of event delegation.

To put it in simple terms: event delegation is the process of registering a handler high up in the DOM tree to handle events happening at lower levels.
Event delegation is made possible by 2 things:

Concretely, instead of just doing some work, a delegated handler will check the target first and then do some work:

    var doWork = function() { console.log('did some work'); }
    var button = document.findElementsByClassName('a-button')[0];

    // NON-DELEGATED version (the handler is bound directly)
    button.addEventListener('click', doWork, false); // use bubbling

    // DELEGATED version (we're checking the target first)
    document.addEventListener('click', function(e) {
        if ( === 'a-button') { // <== "Target check"
    }, false); // use bubbling too

The delegated version above has several advantages:

Of course jQuery is more sophisticated and lets you delegate your handler on a container by specifying a jQuery selector. Under the hoods jQuery will check if the event target matches this jQuery selector. If it does your handler will be called.

DOM Manipulation

DOM manipulations are expensive. Libraries such as jQuery do a very good job of thinking about performance which is why relying on an abstraction to manipulate the DOM makes sense (manipulating the DOM yourself would most likely result in your app being slow and leaking memory). For instance you’re very unlikely to use methods such as createDocumentFragment if you’re authoring a one-off DOM manipulation.

Steps to implement DOM insertion correctly:

Removing elements is tricky because you have to be careful to remove the associated handlers not to create memory leaks.
Cloning elements is difficult in IE because IE copies not only the DOM node but also event handlers (heh.)


If you found this article useful I highly recommend the full book, Secrets Of The JavaScript Ninja. I personally found the book super useful and illuminating in a lot of ways.

It’s good to be able to read about “real-world” JavaScript, including about browser quirks, language pitfalls and testing.