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JAVASCRIPT PDF FOR BEGINNERS WITH EXAMPLES

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When working with the Module pattern, we may find it useful to define a simple template that we use for getting started with it. The module itself is completely self-contained in a global variable called basketModule.

The basket array in the module is kept private and so other parts of our application are unable to directly read it. It only exists with the module's closure and so the only methods able to access it are those with access to its scope i. Notice how the scoping function in the above basket module is wrapped around all of our functions, which we then call and immediately store the return value of. This has a number of advantages including: The freedom to have private functions and private members which can only be consumed by our module.

As they aren't exposed to the rest of the page only our exported API is , they're considered truly private. Given that functions are declared normally and are named, it can be easier to show call stacks in a debugger when we're attempting to discover what function s threw an exception.

J Crowder has pointed out in the past, it also enables us to return different functions depending on the environment. In the past, I've seen developers use this to perform UA testing in order to provide a code-path in their module specific to IE, but we can easily opt for feature detection these days to achieve a similar goal. Module Pattern Variations Import mixins This variation of the pattern demonstrates how globals e.

This effectively allows us to import them and locally alias them as we wish.

This takes as its first argument a dot-separated string such as myObj. Using setObject allows us to set the value of children, creating any of the intermediate objects in the rest of the path passed if they don't already exist.

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For example, if we wanted to declare basket. Here, we see an example of how to define a namespace which can then be populated with a module containing both a private and public API. The following example is heavily based on the original YUI Module pattern implementation by Eric Miraglia, but again, isn't vastly different from the vanilla JavaScript version: Y.

Ben Cherry previously suggested an implementation where a function wrapper is used around module definitions in the event of there being a number of commonalities between modules. In the following example, a library function is defined which declares a new library and automatically binds up the init function to document.

For starters, it's a lot cleaner for developers coming from an object-oriented background than the idea of true encapsulation, at least from a JavaScript perspective. Secondly, it supports private data - so, in the Module pattern, public parts of our code are able to touch the private parts, however the outside world is unable to touch the class's private parts no laughing!

The best free JavaScript resources

Oh, and thanks to David Engfer for the joke. Disadvantages The disadvantages of the Module pattern are that as we access both public and private members differently, when we wish to change visibility, we actually have to make changes to each place the member was used.

We also can't access private members in methods that are added to the object at a later point. That said, in many cases the Module pattern is still quite useful and when used correctly, certainly has the potential to improve the structure of our application.

Other disadvantages include the inability to create automated unit tests for private members and additional complexity when bugs require hot fixes. It's simply not possible to patch privates. Instead, one must override all public methods which interact with the buggy privates. Developers can't easily extend privates either, so it's worth remembering privates are not as flexible as they may initially appear.

For further reading on the Module pattern, see Ben Cherry's excellent in-depth article on it.

The Revealing Module pattern came about as Heilmann was frustrated with the fact that he had to repeat the name of the main object when we wanted to call one public method from another or access public variables. The result of his efforts was an updated pattern where we would simply define all of our functions and variables in the private scope and return an anonymous object with pointers to the private functionality we wished to reveal as public.

It also makes it more clear at the end of the module which of our functions and variables may be accessed publicly which eases readability. Disadvantages A disadvantage of this pattern is that if a private function refers to a public function, that public function can't be overridden if a patch is necessary.

This is because the private function will continue to refer to the private implementation and the pattern doesn't apply to public members, only to functions.

Public object members which refer to private variables are also subject to the no-patch rule notes above. As a result of this, modules created with the Revealing Module pattern may be more fragile than those created with the original Module pattern, so care should be taken during usage. The Singleton Pattern The Singleton pattern is thus known because it restricts instantiation of a class to a single object. Classically, the Singleton pattern can be implemented by creating a class with a method that creates a new instance of the class if one doesn't exist.

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In the event of an instance already existing, it simply returns a reference to that object. Singletons differ from static classes or objects as we can delay their initialization, generally because they require some information that may not be available during initialization time.

They don't provide a way for code that is unaware of a previous reference to them to easily retrieve them. This is because it is neither the object or "class" that's returned by a Singleton, it's a structure.

Think of how closured variables aren't actually closures - the function scope that provides the closure is the closure. In JavaScript, Singletons serve as a shared resource namespace which isolate implementation code from the global namespace so as to provide a single point of access for functions.

What makes the Singleton is the global access to the instance generally through MySingleton. This is however possible in JavaScript.

In the GoF book, the applicability of the Singleton pattern is described as follows: There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. When the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code.

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The second of these points refers to a case where we might need code such as: mySingleton. FooSingleton above would be a subclass of BasicSingleton and implement the same interface. Why is deferring execution considered important for a Singleton?

It is important to note the difference between a static instance of a class object and a Singleton: whilst a Singleton can be implemented as a static instance, it can also be constructed lazily, without the need for resources nor memory until this is actually needed. If we have a static object that can be initialized directly, we need to ensure the code is always executed in the same order e. Both Singletons and static objects are useful but they shouldn't be overused - the same way in which we shouldn't overuse other patterns.

In practice, the Singleton pattern is useful when exactly one object is needed to coordinate others across a system. They're often an indication that modules in a system are either tightly coupled or that logic is overly spread across multiple parts of a codebase. If you try all the examples, you will learn a lot about JavaScript, in a very short time! Why Study JavaScript?

JavaScript is one of the 3 languages all web developers must learn: 1. HTML to define the content of web pages 2. CSS to specify the layout of web pages 3. JavaScript to program the behavior of web pages Web pages are not the only place where JavaScript is used. Many desktop and server programs use JavaScript.

Did You Know? JavaScript and Java are completely different languages, both in concept and design. ECMA is the official name of the standard.For example, when an object needs to be able to notify other objects without making assumptions regarding those objects.

See below for inline comments on what these components do in the context of our example. When working with JavaScript, different errors can occur. There is no better time to learn these JavaScript libraries.

Disadvantages The disadvantages of the Module pattern are that as we access both public and private members differently, when we wish to change visibility, we actually have to make changes to each place the member was used.

The result of his efforts was an updated pattern where we would simply define all of our functions and variables in the private scope and return an anonymous object with pointers to the private functionality we wished to reveal as public. Redemption November 1, at ECMA is the official name of the standard. The DOM represents the document as nodes and objects. Copyright by Refsnes Data.