explain this javascript function declaration "jQuery(function($){}"

Question

i came across this piece of script in the source code of a chrome extension. it declares a function:

jQuery(function($){
 $('#actionCopy').click(function(e){
 Action.copy();
 });
});

what is that jQuery word used at the beginning of the function? and what does the dollar sign that is used as the function argument do? ( function($) )

• Fabrício Matté: And you didn't even tag this with jQuery sigh. Anyway the most complete answer will be found on this link.
• Felix Kling: api.jquery.com/jQuery/#jQuery3
• I Hate Lazy: "what is that jQuery word used at the beginning of the function?" Well, it's the name of a function being invoked jQuery(/**args**/) just like you'd expect for any other function. The argument passed to the jQuery() function just happens to be another function. Functions are objects and can be passed around just like any other data.
• I Hate Lazy: ...in a similar manner, the $ is a parameter to the function argument. When invoked it is passed a value. That value is a function. The function argument passed to $ just so happens to be the same jQuery function, so now you have two different references to the same function. jQuery === $; // true
• burhan: is it the same as this? (function($){ //code })(jQuery);

Answer 1

Passing in a function as first argument into the jQuery constructor function is just a shortcut for .ready().

jQuery(document).ready(function() {
});

jQuery will be kind enough to pass in the jQuery object reference into the callback which you pass in for the .ready handler. That means, you can just savely access the jQuery object using the dollar sign $ within.

Answer 2

what is that jQuery word used at the beginning of the function?

It refers to a function called jQuery (defined by the library available from http://jquery.com).

That particular function is horribly overloaded, but in this case (when it receives one argument that is a function) it means "Bind this function to the document ready event" (with some pollyfill for the benefit of browsers that don't support that event natively).

and what does the dollar sign that is used as the function argument do?

The same as a letter a or the word foo or anything else. It defines the variable name that the argument will be passed to. There is nothing special about the $ character in JavaScript variable names.

Answer 3

jQuery is popular javascript library. $ is the shorthand for accessing jQuery object. Passing $ in function 'function($)' will make sure that $ will not be conflicted with any other javascript library used.