为什么要定义一个匿名函数并将jQuery作为参数传递给它？

Question

I'm looking through the excellent peepcode demo code from the backbone.js screencasts. In it, the backbone code is all enclosed in an anonymous function that is passed the jQuery object:

(function($) {
  // Backbone code in here
})(jQuery);

In my own backbone code, I've just wrapped all my code in the jQuery DOM 'ready' event:

$(function(){
  // Backbone code in here
});

What's the point/advantage of the first approach? Doing it this way creates an anonymous function that is then executed immediately with the jQuery object being passed as the function argument, effectively ensuring that $ is the jQuery object. Is this the only point - to guarantee that jQuery is bound to '$' or are there other reasons to do this?

Answer 1

The two blocks of code you have shown are dramatically different in when and why they execute. They are not exclusive of each other. They do not serve the same purpose.

JavaScript Modules

(function($) {
  // Backbone code in here
})(jQuery);

This is a "JavaScript Module" pattern, implemented with an immediately invoking function.

• http://addyosmani.com/resources/essentialjsdesignpatterns/book/#modulepatternjavascript
• http://www.adequatelygood.com/2010/3/JavaScript-Module-Pattern-In-Depth

The purpose of this code is to provide "modularity", privacy and encapsulation for your code.

The implementation of this is a function that is immediately invoked by the calling (jQuery) parenthesis. The purpose of passing jQuery in to the parenthesis is to provide local scoping to the global variable. This helps reduce the amount of overhead of looking up the $ variable, and allows better compression / optimization for minifiers in some cases.

Immediately invoking functions are executed, well, immediately. As soon as the function definition is complete, the function is executed.

jQuery's "DOMReady" function

This is an alias to jQuery's "DOMReady" function: http://api.jquery.com/ready/

$(function(){
  // Backbone code in here
});

jQuery's "DOMReady" function executes when the DOM is ready to be manipulated by your JavaScript code.

Modules vs DOMReady In Backbone Code

It's bad form to define your Backbone code inside of jQuery's DOMReady function, and potentially damaging to your application performance. This function does not get called until the DOM has loaded and is ready to be manipulated. That means you're waiting until the browser has parsed the DOM at least once before you are defining your objects.

It's a better idea to define your Backbone objects outside of a DOMReady function. I, among many others, prefer to do this inside of a JavaScript Module pattern so that I can provide encapsulation and privacy for my code. I tend to use the "Revealing Module" pattern (see the first link above) to provide access to the bits that I need outside of my module.

By defining your objects outside of the DOMReady function, and providing some way to reference them, you are allowing the browser to get a head start on processing your JavaScript, potentially speeding up the user experience. It also makes the code more flexible as you can move things around without having to worry about creating more DOMREady functions when you do move things.

You're likely going to use a DOMReady function, still, even if you define your Backbone objects somewhere else. The reason is that many Backbone apps need to manipulate the DOM in some manner. To do this, you need to wait until the DOM is ready, therefore you need to use the DOMReady function to start your application after it has been defined.

You can find plenty of examples of this around the web, but here's a very basic implementation, using both a Module and the DOMReady function:

// Define "MyApp" as a revealing module

MyApp = (function(Backbone, $){

  var View = Backbone.View.extend({
    // do stuff here  
  });

  return {
    init: function(){
      var view = new View();
      $("#some-div").html(view.render().el);
    }
  };

})(Backbone, jQuery);



// Run "MyApp" in DOMReady

$(function(){
  MyApp.init();
});

Answer 2

As a minor sidenote, sending in $ as an argument to an anonymous function makes $ local to that function which has a small positive performance implication if the $ function is called a lot. This is because javascript searches the local scope for variables first and then traverses down all the way to the window scope (where $ usually lives).

Answer 3

It ensures you can always use $ inside that closure even if $.noConflict() was used.

Without this closure you'd be supposed to use jQuery instead of $ the whole time.

Answer 4

It is to avoid a potential conflict of the $ variable. If something else defines a variable named $, your plugin may use the wrong definition

Refer to http://docs.jquery.com/Plugins/Authoring#Getting_Started for more details

Answer 5

Use both.

The self invoking function in which you pass in jQuery to prevent library conflicts, and to just make sure jQuery is available as you would expect with $.

And the .ready() shortcut method as required to run javascript only after DOM has loaded:

(function($) {
    $(function(){
          //add code here that needs to wait for page to be loaded
    });

    //and rest of code here
})(jQuery);