1. JQuery selects web elements

The basic design idea and main usage of jQuery is to “select a web element and do something with it”. This is the fundamental feature that sets it apart from other Javascript libraries. The first step in using jQuery is often to put a selection expression into the constructor jQuery() ($for short) and get the selected element. Select expressions can be CSS selectors:

$(document) // Select the entire document object $('#myId') // select the page element $('div. MyClass ') // select the div element whose class is myClass $('input[name=first]') // Select the input element whose name attribute equals firstCopy the code

It can also be a jquery-specific expression:

$('a.first') // Select the first a element in the page $('tr:ood') // odd rows in the table $('#myForm :input') // Select the input element in the form $('div:visible') // Select the visible div element $('div:gt(2)') // Select all div elements except the first three $('div:animated') // select the div elements that are currently animatedCopy the code

2. Change the result set

The second idea behind jQuery is to provide a variety of powerful filters to filter the structure and narrow the selection:

$('div').has('p'); // Select div element $('div').not('.myclass '); // select the div element $('div').filter('.myClass'); // select the div element $('div').first(); // Select the first div element $('div').eq(5); // Select the sixth div elementCopy the code

Sometimes we need to start from a result set and move to a nearby related element. JQuery also provides a way to move around the DOM tree:

$('div').next('p'); $('div').parent(); $('div').parent(); Closest ('form'); // Select the form parent element $('div').children(); $('div').siblings(); // Select the sibling element of divCopy the code

3. Chain operation

JQuery design idea 3, is the final selection of web elements, you can make a series of operations on it, and all operations can be linked together, in the form of a chain, for example:

$('div').find('h3').eq(2).html('Hello');

The breakdown is as follows:

$(' div ') / / find div elements. The find (' h3) / / choice of h3 elements. The eq. (2) / / select 3 h3 elements. The HTML (" Hello "); // Change its content to Hello to indicate that h1 is assignedCopy the code

This is the most flattering and convenient feature of jQuery. It works because each step of the jQuery operation returns a jQuery object, so different operations can be linked together. JQuery objects are created by jQuery functions and are elements corresponding to object operations.

JQuery also provides the.end() method, which allows the result set to take a step back:

. Find (' h3). Eq. (2). The HTML (' Hello '.) the end () / / return to h3 elements of the selected all of the step. The eq (0) / / selected first h3 elements. The HTML (" World "); // Change its content to WorldCopy the code

4. Operations on elements

  • 4.1 Value and assignment

The fourth design idea of jQuery is to use the same function for getters and setters. Whether to evaluate or assign depends on the parameters of the function.

$('h1').html(); $('h1').html('Hello'); // HTML () takes the argument Hello, which assigns h1Copy the code

Common values and assignment functions are as follows:

.html() takes or sets the HTML content.text() Takes or sets the text content.attr() takes or sets the value of an attribute.width() Takes or sets the width of an element.height() Takes or sets the height of an element .val() retrieves the value of a form elementCopy the code

Note that if the result set contains more than one element, then all elements are assigned; Fetching only the value of the first element (except.text(), which fetches the text content of all elements).

  • 4.2 Operation of elements: move

The fifth design idea of jQuery is to provide two sets of methods for manipulating the position of elements in a web page. One set of methods is to move the element directly, and the other set of methods is to move other elements so that the target element is where we want it to be.

Suppose we select a div element and need to move it after the P element.

The first method is to use.insertafter (), which moves the div element after the p element:

$(‘div’).insertAfter($(‘p’)); The second way is to use.after(), which precedes the p element in the div:

$('p').after($('div'));

On the surface, the effect of the two methods is the same, the only difference seems to be the perspective of operation. But in fact, there is one big difference between them, and that is that they return different elements. The first method returns the div element, and the second method returns the P element. You can choose which method to use based on your needs.

There are four pairs of operation methods using this mode:

.insertafter () and.after() : inserts elements.insertbefore () and.before() : inserts elements.appendto () and.append() from the front outside the existing element. Inside the existing element, insert elements from behind. PrependTo () and. Prepend () : Inside the existing element, insert elements from the frontCopy the code
  • 4.3 Operations on elements: Copy, delete, and create

In addition to moving elements around, jQuery provides several other important ways to manipulate elements.

Copy elements using.clone().

Remove elements using.remove() and.detach(). The difference between the two is that the former does not hold the event of the deleted element. The latter is reserved for use when re-inserting the document.

Empty the element (but not delete it) using.empty().

Creating a new element is as simple as passing it directly into the jQuery constructor:

$(' < p > Hello < / p > '); $(' < li class = "new" > new list item < / li > '); $('ul').append('<li>list item</li>');Copy the code

5. Tools and methods

In addition to operating on selected elements, jQuery also provides some element-independent utility methods. You can use these methods directly without having to select the element.

If you understand the inheritance principles of the Javascript language, you can understand the nature of the tool approach. It is a method defined on the jQuery constructor, jquery.method (), so it can be used directly. The methods that operate on elements are those defined on the constructor’s prototype object, jquery.prototype.method (), so you have to generate instances (that is, select elements) to use. If you don’t understand this distinction, it’s ok to think of utility methods as methods that can be used directly, like javascript native functions.

The following tools are commonly used:

$.trim() removes Spaces at both ends of the string. $.each() iterates over an array or object. $.inarray () returns the index position of a value in the array. If the value is not in the array, -1 is returned. $.grep() returns the elements in the array that meet certain criteria. $.extend() merges multiple objects into the first object. $.makeArray() converts an object to an array. $.type() determines the class of objects (function objects, date objects, array objects, re objects, and so on). $.isarray () checks whether a parameter is an array. $.isemptyObject () determines whether an object is empty (without any attributes). $.isfunction () checks whether an argument is a function. $.isplainObject () determines whether a parameter is an Object created with "{}" or "new Object". $.support() Checks whether the browser supports a feature.Copy the code

For more information about jQuery, see:Ruan Yifeng design ideas / JQuery API Documentation in Chinese