Question

$.each(obj,function(key,value) {})

I am a newbie of jquery, i know there are two ways to write the structure like this:

$('document').ready(function(){}); or     $(function(){});

why $.each() put . behind $ rather than $(each())

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Answer 1

In JavaScript (and remember, jQuery is a JavaScript library that doesn’t do anything you couldn’t do yourself in JavaScript) functions are a type of object. Hopefully you already know how to define and call a JavaScript function, with the following syntax that is similar to many other languages:

function myFunction() {
   alert("This is myFunction");
}

myFunction();      // alerts "This is myFunction"

But because a function is also an object it can have other properties:

myFunction.someProperty = "test";
alert(myFunction.someProperty);    // alerts "test"

In that example someProperty was a simple string, but you can also create properties that refer to functions, i.e., that are methods:

myFunction.someMethod = function() {
                          alert("This is someMethod");
                        };

myFunction.someMethod();    // alerts "This is someMethod"

The jQuery syntax you have asked about is essentially the same thing as above, except at first it seems more complicated because the $() function does different things depending on the type of the parameter(s) you pass to it. But

$(function(){});
// is equivalent to my example
myFunction();

$.each();
// is equivalent to my example
myFunction.someMethod();

Some jQuery code gets a bit more confusing because in many cases jQuery function calls return a “jQuery object” that itself has a bunch of methods defined.

$(document)
// returns a jQuery object that has a "ready" method so you can say
$(document).ready();
// which is equivalent to 
var doc = $(document);
doc.ready();

Many jQuery methods return the same type of jQuery object, so you can chain calls together:

$('div').hide().fadeIn().fadeOut().show();

The last .show() at the end also returns a jQuery object but we aren’t doing anything with it.

These concepts are all a standard part of the JavaScript language, so I suggest you work your way through a good JavaScript tutorial. If you look at the jQuery source code (i.e., the content of jQuery.js) there is a lot of confusing code to digest, but if you start by playing around with something simple like my first few examples above you’ll get the hang of it.