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What does this JavaScript/jQuery syntax mean?
I specifically mean when you do this:
(function ($) {
...
})(jQuery);
I've never seen that kind of syntax before. How does the function get called? I understand when you do it like this:
jQuery(function ($) {
...
});
because the function is being passed to jQuery, and jQuery can just run any function passed as a parameter when the DOM's ready, but the first one's different.
Duplicate of What does this JavaScript/jQuery syntax mean?
I'll post my answer here, though seeing as Jeff Attwood seems to want us to embrace duplication: (https://blog.stackoverflow.com/2010/11/dr-strangedupe-or-how-i-learned-to-stop-worrying-and-love-duplication/)
This convention is used when writing plugins to ensure there is no confilict with other Javascript libraries using the $ notation, whilst ensuring the plugin author can still use this notataion:
(function($){
...
})(jQuery);
The author is declaring an anonymous function with a single parameter ($), then immediately calling it and passing the jQuery object to it. This ensures the function is called and that everything in it is defined.
A longer notation might be:
function MyDefs($){
...
}
MyDefs(jQuery);
Although that would create a variable MyDefs in the global namespace. The anonymous function pattern leaves the global namespace empty, avoiding conflicts.
It's an anonymous function. When you write:
(function ($){
..
})(jQuery);
It is mostly equivalent to:
function the_function($) {
..
}
the_function(jQuery);
The only difference being that the first does not create a function called the_function and therefore created no risk of accidentally overwriting an existing function or variable with that name. And of course, all of it is equivalent to:
function the_function() {
var $ = jQuery;
..
}
the_function();
The point of this construct is that any variables defined inside the_function are local and therefore cannot accidentally overwrite any variables or functions in the global scope. For instance, the code inside the function uses $ to represent the jQuery object, but this would conflict with other libraries that use $ as well (such as Prototype). By wrapping the usage of $ inside a function, the Prototype code outside the function remains unaffected.
Related
This question already has answers here:
What is the purpose of this? (function ($) { //function code here })(jQuery);
(4 answers)
Closed 9 years ago.
I noticed that in some places, jQuery code is wrapped in a self-invoking function like below. Why is this done, in what cases is this useful and in what cases is an unnecessary boilerplate?
function( $ ) {
...
}( jQuery );
The short answer: To prevent variable name conflicts. It is not always needed, but good practice in order to create conflict free reusable code.
The long answer: In javascript the $ symbol is just another variable. jQuery uses it because it is a nice shorthand instead of having to type out jQuery each time, but so can any other code (including other libraries).
To prevent conflict with other uses of a variable at the same scope (in this case $ at the "global" scope) it is common for code to be wrapped in a self-invoking function with the "conflict-free" variables passed as the function parameters. This then creates a new scope where your variable won't interfere with other uses of that variable. That way you can pass the full named variable & uses it with whatever name you want within the anonymous function.
So, when creating conflict free reusable code it is good practice to use this methodology:
(function( $ ) {
...
})( jQuery );
Along these lines you will also see the following format:
(function( $, window, undefined ) {
...
})( jQuery, window );
In this instance undefined is simply used for readability to indicating that no more arguments are passed to the function.
In case you want to avoid conflict regarding use of $
function( $ ) {
...
}( jQuery );
Inside this function you can use $ without having to worry about it's use outside of it as inside that function, $ always refers to the jQuery object.
This is helpful while creating jQuery plugins,You will see jQuery plugin's use this kind of function to avoid conflicts with other plugins.
Reference : http://learn.jquery.com/plugins/basic-plugin-creation/
In function scope $ is local variable that not conflict with any other global $.
This question already has answers here:
What is the purpose of a self executing function in javascript?
(21 answers)
Closed 8 years ago.
So far I've learned the benefits of using this function (is it wrapping?)
So, it almost acts like namespaces.
Suppose we have:
( function() {
function foo(){
alert(true);
}
foo(); //alerts true
})();
( function() {
function foo(){ //the same title of the function as above
alert("another call of foo");
}
foo(); //alerts, ok.
})();
Also I've noticed it can access public vars, like this:
var __foo__ = 'bar';
( function() {
alert(__foo__); //alerts bar
})();
I have several questions regarding this approach
What I've tried:
Use Bing for tutorials (I' found them, but many of them don't answer my questions)
Play with passing objects into the body
Find the answer here
But, I'm still beating my head against the wall
So the questions are:
I've seen people pass objects as params, but when DOES it make sense?
For example, what does it mean?
( function(window) {
})(document);
I saw smth like this in Jquery UI Lib
( function($) {
//some code of widget goes here
})(Jquery);
This makes inner code visible outside the function, right? (not sure) Why, this is because
we can access the object (say we have "modal" widget), simply by calling it,
like:
$(function(){
$("#some_div").modal(); //here it's object the we got from the function
});
And the second question is: How does it work.
I've seen people pass objects as params, but when DOES it make sense? For example, what does it mean?
( function(window) {
})(document);
The language does not treat parameters to immediately called functions differently than parameters to other functions.
It makes sense to use a parameter whenever you want a local name in your function body for an input. In this case it's a bit confusing since window and document are likely to be confused.
( function($) {
//some code of widget goes here
})(Jquery);
This makes inner code visible outside the function, right? (not sure) Why, this is because we can access the object (say we have "modal" widget), simply by calling it,
No. It does not by itself make any code visible outside the widget. It's just a parameter definition which provides a new&local name for a global variable.
What makes inner code visible outside is attaching it to an external object as in
$.exportedProperty = localVariable;
which is a common convention in jQuery code.
There are mainly 2 purposes of passing in the window and document objects such as seen below
(function(window, document){
// code
}(window, document);
Javascript can access local variables faster than global variables. This pattern in effect makes the names window and document local variables rather than global, thus making your script slightly faster.
Making these names local variables has another benefit: minifiers can rename them. So if you minify the above script, the local version of window might get renamed to a and document might get renamed to b, thus making the minified script smaller. If you were to reference them as globals, these renamings are impossible because that would break your script.
For more info, checkout these awesome videos
http://paulirish.com/2010/10-things-i-learned-from-the-jquery-source/
http://paulirish.com/2011/11-more-things-i-learned-from-the-jquery-source/
on your first question, I dont think you seen window and document but something more like:
(function(doc) {
var fubar = doc.getElementById("fubar"); // === document.getElementById("fubar")
})(document);
you have a self-invoking function (or closure) with arguments like any function:
var b = function(str) { alert(str); }
b('hi there') //alert('hi there');
the same thing is it with the code above, but we are just calling the method at once we created it.
the other code you have:
( function($) {
//some code of widget goes here
})(Jquery);
is to reserve the $variable inside the metod to refer to the jQuery object, this is very handy if you have more frameworks or replaced the $ object with something else, everything inside that method with an $ will refer to the jQuery object and nothing else(if you don´t replace it inside your code).
the code:
$(function(){
$("#some_div").modal(); //here it's object the we got from the function
});
is calling jQuery and its a shortcut for $(document).ready
it will call the method:
function(){
$("#some_div").modal(); //here it's object the we got from the function
}
as soon as the DOM is ready
The pattern is called a closure. It makes sense to use when a module or function:
wants to avoid polluting globally-scoped variables
wants to avoid use globally-scoped variables and avoid other code polluting them
For an example of each, first take this pattern:
(function(window) {
// ...
})(window);
Everything inside the closure will be able to use window as if it were a local variable.
Next, take the same pattern using the JQuery symbol:
(function($) {
// ...
})($);
If you have some code that relies on a symbol/namespace like $, but another module reassigns that, it can screw up your code. Using this pattern avoids this by allowing you to inject the symbol into a closure.
Whenever you pass an argument to that wrapping function it's so that you won't mess up with any other libraries or global variables that may be present in your application.
For example as you may know jQuery uses $ as a symbol for calling itself, and you may also have another library, that will also use $ for calling itselt, under this condition you may have trouble referencing your libraries. so you would solve it like this:
(function($){
// here you're accessing jQuery's functions
$('#anything').css('color','red');
})(jQuery);
(function($){
// and in here you would be accessing the other library
$.('#anything').method();
})(otherLibrary);
This is specially useful when you're making jQuery or any other kind of library plugins.
What it does is allow you to use the $ variable inside your function in place of the jQuery variable, even if the $ variable is defined as something else outside your function.
As an example, if you're using both jQuery and Prototype, you can use jQuery.noConflict() to ensure that Prototype's $ is still accessible in the global namespace, but inside your own function you can use $ to refer to jQuery.
I have a script which contains the following piece of code:
(function ($) {
// ...
})($);
Can anyone explain how to read it?
Create an anonymous function
Let it take one argument which will be called $
Call the function immediately
Pass it one argument which is whatever the value of $ is in the outer scope.
If the $ in the outer scope changes (by having a new value assign to it) then the value for $ in the inner scope will be protected from the change (since it is a different variable).
This also provides a clean scope for all other variables declared inside the function (if they use var as they should).
'$' is a legal variable in JavaScript. It could have just as easily been someCrazyLongVariableName
It kind of defeats the purpose of encapsulating the $ sign into an anonymous function.
It's a self executing anonymous function that takes $ as argument.
In most of the cases, you use this markup to avoid js libraries collisions : say that you want to use jQuery and prototype in your project. Very nice, but they both use the '$' sign... how to overcome this?
// jquery code here
(function($){
alert($ === jQuery);// the $ is a shortcut to jQuery
})(jQuery);
// non-jquery code here - ex: prototype
alert($ === jQuery);// the $ is NOT a shortcut to jQuery ANYMORE
If you're not thinking about the $ in particular, the extract is creating a javascript function and calling it in a single statement.
I'm not 100% sure, but i guess that the 2 $ variables would be a different scope.
I found this page useful, if a little verbose: -
http://www.authenticsociety.com/blog/JavaScript_DollarSign
In summary, it's no different to calling the variable a, but should be avoided.
The jQuery site lists the basic plugin syntax for jQuery as this:
(function( $ ){
$.fn.myPlugin = function() {
// there's no need to do $(this) because
// "this" is already a jquery object
// $(this) would be the same as $($('#element'));
this.fadeIn('normal', function(){
// the this keyword is a DOM element
});
};
})( jQuery );
I'd just like to understand what is going on there from Javascript's point of view, because it doesn't look like it follows any syntax I've seen JS do before. So here's my list of questions:
If you replace function($)... with a variable, say "the_function", the syntax looks like this:
(the_function)( jQuery );
What is "( jQuery );" doing? Are the parenthesis around the_function really necessary? Why are they there? Is there another piece of code you can give that is similar?
It begins with function( $ ). So it's creating a function, that as far as I can tell will never be run, with the parameter of $, which is already defined? What is going on there?
Thanks for the help!
function(x){
x...
}
is just a function without a name, that takes one argument, "x", and does things with x.
Instead of 'x', which is a common variable name, you can use $, which is a less common variable name, but still legal.
function($){
$...
}
I'll put it in parentheses to make sure it parses as an expression:
(function($){
$....
})
To call a function, you put () after it with a list of arguments. For example, if we wanted to call this function passing in 3 for the value of $ we would do this:
(function($){
$...
})(3);
Just for kicks, let's call this function and pass in jQuery as a variable:
(function($){
$....
})(jQuery);
This creates a new function that takes one argument and then calls that function, passing in jQuery as the value.
WHY?
Because writing jQuery every time you want to do something with jQuery is tedious.
WHY NOT JUST WRITE $ = jQuery?
Because someone else might have defined $ to mean something else. This guarantees that any other meanings of $ are shadowed by this one.
(function( $ ){
})( jQuery );
That is self-executing anonymous function that uses $ in argument so that you can use it ($) instead of jQuery inside that function and without the fear of conflicting with other libraries because in other libraries too $ has special meaning. That pattern is especially useful when writing jQuery plugins.
You can write any character there instead of $ too:
(function(j){
// can do something like
j.fn.function_name = function(x){};
})(jQuery);
Here j will automatically catch up jQuery specified at the end (jQuery). Or you can leave out the argument completely but then you will have to use jQuery keyword all around instead of $ with no fear of collision still. So $ is wrapped in the argument for short-hand so that you can write $ instead of jQuery all around inside that function.
If you even look at the source code of jQuery, you will see that everything is wrapped in between:
(function( window, undefined ) {
// jQuery code
})(window);
That is as can be seen also a self-executing anonymous function with arguments. A window (and undefined) argument is created and is mapped with global window object at the bottom (window). This is popular pattern these days and has little speed gain because here window is will be looked into from the argument rather than global window object which is mapped below.
The $.fn is jQuery's object where you create your new function (which is also an object) or the plugin itself so that jQuery wraps your plugin in its $.fn object and make it available.
Interestingly, I had answered similar question here:
JavaScript / jQuery closure function syntax
You can also check out this article to know more about self-executing functions that I had written:
Javascript Self-executing Functions
The basic plugin syntax allows you to use $ to refer to jQuery in the body of your plugin, regardless of the identify of $ at the time the plugin is loaded. This prevents naming conflicts with other libraries, most notably Prototype.
The syntax defines a function that accepts a parameter known as $ so you can refer to it as $ in the function body, and then immediately invokes that function, putting jQuery in as the argument.
This also helps not pollute the global namespace (so declaring var myvar = 123; in your plugin body won't suddenly define window.myvar), but the main ostensible purpose is to allow you to use $ where $ may have since been redefined.
You're dealing with a self-invoking anonymous function there. It's like "best practice" to wrap a jQuery plugin within such a function to make sure, that the $ sign is bound to the jQuery object.
Example:
(function(foo) {
alert(foo);
}('BAR'));
This would alert BAR when put into a <script> block. The parameter BAR is passed to the function which calls itself.
The same principle is happening in your code, the jQuery object is passed to the function, so $ will refer to the jQuery object for sure.
The jQuery at the end passes itself (jQuery) over to the function, so that you can use the $ symbol within your plugin. You ccould also do
(function(foo){
foo.fn.myPlugin = function() {
this.fadeIn('normal', function(){
});
};
})( jQuery );
To find a clear explanation of this and other modern javascript tricks and common practices, I recommend reading Javascript Garden.
http://bonsaiden.github.com/JavaScript-Garden/
It's especially useful, because many of these patterns are widely used in many libraries but not really explained.
The other answers here are great, but there is one important point that hasn't been addressed. You say:
So it's creating a function, that as far as I can tell will never be run, with the parameter of $, which is already defined?
There is no guarantee that the global variable $ is available. By default, jQuery creates two variables in the global scope: $ and jQuery (where the two are aliases for the same object). However, jQuery can also be run in noConflict mode:
<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript">
jQuery.noConflict();
</script>
When you call jQuery.noConflict(), the global variable $ is set back to whatever it was before the jQuery library was included. This allows jQuery to be used with other Javascript libraries that also use $ as a global variable.
If you wrote a plugin that relied on $ being an alias for jQuery, then your plugin would not work for users running in noConflict mode.
As others have already explained, the code you posted creates an anonymous function that is called immediately. The global variable jQuery is then passed in to this anonymous function, which is safely aliased as the local variable $ within the function.
what does this mean?
(function($){
})(jQuery);
to make the question clearer, what does wrapping a function in parenthesis mean in JS (sorry, I'm a bit confused on the concept of closures). What about the $ parameter? and the "jQuery" in the end parenthesis?
Can I do the same with mootools and combine them in 1 JS file?
(function($){})(jQuery);
(function($){})(mooTools);
I have only worked with jquery and am planning to work with Mootools
Wrapping a function between parenthesis ensures this function to be evaluated as a function expression.
That happens because the Grouping Operator (the parentheses), can only evaluate expressions.
If no parenthesis are used, it will be interpreted as a function declaration, and it will cause a syntax error, since the function name is not optional for function declarations.
(function(arg){
alert(arg); // alerts test
})("test");
In the above example, the function expression is automatically executed, passing an argument.
That pattern is heavily used by jQuery plugins, since jQuery can run in noConflict mode, the $ global variable will not be created, so the jQuery global object is passed as an argument of this anonymous function and inside of that function, you can freely refer to it as $ (the received argument).
Keep in mind that also, the function context (the this keyword) inside self-executing function expressions invoked like the above example, will refer always to the Global object.
For a more in-depth info about the differences between function expressions and function declarations, give a look to the following resources:
Explain JavaScript’s encapsulated anonymous function syntax
Named function expressions demystified
CMS have given you the correct answer but I just want to add that this is not a closure. This is just the () operator being used to return the result of an expression which in this case is a function expression and the fact that in javascript, a returned anonymous function can be called directly. So this is simply combining both:
var x = (1+1); // <-- () evaluates an expression
and:
var arr = [0,1,2];
arr.push(function(text){alert(text)});
arr[3]('hello'); // <-- function returned by array called directly
And as for the $ parameter, that's just one of the characters that javascript allows for variable names. Your examples is exactly equivalent to:
(function(jQ){})(jQuery);
(function(moo){})(mooTools);
The main point for doing this is so that objects created within the function's expression can be utilized from properties and methods attached to passed in objects, without exposing those objects publicly. This can aid in preventing variable collisions. It also allows for variables created to be accessed as private storage for anything that is attached..
(function($){
//jQuery is available here as $
var myPrivateArray = []; //this is private
$.addItem = function(item) {
myPrivateArray.push(item);
}
})(jQuery);
//I can't access myPrivateArray here...
//but I can use jQuery.addItem to add something.
Here is an article on closures: http://www.jibbering.com/faq/faq_notes/closures.html
Basically, as CMS has said, it is a function expression. There is a perfect example to help you better understand about 2/3 of the way down that page.
The jQuery in the last set of parentheses means you're passing the jQuery object to the inner function. That inner function takes the object and it is assigned as $. So, when you're accessing the $ inside the function, you're actually acting upon the jQuery object you've passed.
As for mixing these with jQuery and Mootools, I've never used Mootools so I'm not sure how it assigns itself. The only thing I would think is that if it uses $ like jQuery, you can call jQuery.noConflict(); and reassign jQuery to another variable, maybe var $j = jQuery; Then, you can use Mootools as you normally would.
function($) { ... } creates a function that accepts a parameter called $.
Wrapping in in parentheses does nothing.
Adding (jQuery) calls the function with jQuery as a parameter.
This is done to make the function independent of $ in the global scope.
When you write $ in the function, you're referring to the parameter $, not the global variable $.
the mootools equivalent for namespacing $ is:
(function($) {
// $ is the mootools one (aliasing document.id)
})(document.id);
you can combine them into one file, but keep in mind that mootools is a prototypical framework and $ only serves the purpose of a selector whereas all functions go into the element/array/class etc prototypes. hence doing this in mootools works:
var foo = $("bar");
foo.hide(); // the element inherits .hide()
but in jquery it will fail and needs to be transcribed as
var foo = $("#bar");
$(foo).hide();
the point is, you can't namespace the stuff that mootools exports into prototypes.