jquery基礎（五）

Manipulating Elements

Once you've made a selection, the fun begins. You can change, move, remove, and clone elements. You can also create new elements via a simple syntax.

For complete documentation of jQuery manipulation methods, visithttp://api.jquery.com/category/manipulation/.

Getting and Setting Information about Elements

There are any number of ways you can change an existing element. Among the most common tasks you'll perform is changing the inner HTML or attribute of an element. jQuery offers simple, cross-browser methods for these sorts of manipulations. You can also get information about elements using many of the same methods in their getter incarnations. We'll see examples of these throughout this section, but specifically, here are a few methods you can use to get and set information about elements.

Note

Changing things about elements is trivial, but remember that the change will affect all elements in the selection, so if you just want to change one element, be sure to specify that in your selection before calling a setter method.

Note

When methods act as getters, they generally only work on the first element in the selection, and they do not return a jQuery object, so you can't chain additional methods to them. One notable exception is $.fn.text; as mentioned below, it gets the text for all elements in the selection.

$.fn.html

Get or set the html contents.

$.fn.text

Get or set the text contents; HTML will be stripped.

$.fn.attr

Get or set the value of the provided attribute.

$.fn.width

Get or set the width in pixels of the first element in the selection as an integer.

$.fn.height

Get or set the height in pixels of the first element in the selection as an integer.

$.fn.position

Get an object with position information for the first element in the selection, relative to its first positioned ancestor. This is a getter only.

$.fn.val

Get or set the value of form elements.

Example 3.26. Changing the HTML of an element

$('#myDiv p:first')
    .html('New <strong>first</strong> paragraph!');

Moving, Copying, and Removing Elements

There are a variety of ways to move elements around the DOM; generally, there are two approaches:

Place the selected element(s) relative to another element

Place an element relative to the selected element(s)

For example, jQuery provides $.fn.insertAfter and $.fn.after. The $.fn.insertAftermethod places the selected element(s) after the element that you provide as an argument; the$.fn.after method places the element provided as an argument after the selected element. Several other methods follow this pattern: $.fn.insertBefore and $.fn.before;$.fn.appendTo and $.fn.append; and $.fn.prependTo and $.fn.prepend.

The method that makes the most sense for you will depend on what elements you already have selected, and whether you will need to store a reference to the elements you're adding to the page. If you need to store a reference, you will always want to take the first approach -- placing the selected elements relative to another element -- as it returns the element(s) you're placing. In this case,$.fn.insertAfter, $.fn.insertBefore, $.fn.appendTo, and $.fn.prependTo will be your tools of choice.

Example 3.27. Moving elements using different approaches

// make the first list item the last list item
var $li = $('#myList li:first').appendTo('#myList');

// another approach to the same problem
$('#myList').append($('#myList li:first'));

// note that there's no way to access the
// list item that we moved, as this returns
// the list itself

Cloning Elements

When you use methods such as $.fn.appendTo, you are moving the element; sometimes you want to make a copy of the element instead. In this case, you'll need to use $.fn.clone first.

Example 3.28. Making a copy of an element

// copy the first list item to the end of the list
$('#myList li:first').clone().appendTo('#myList');

Note

If you need to copy related data and events, be sure to pass true as an argument to $.fn.clone.

Removing Elements

There are two ways to remove elements from the page: $.fn.remove and $.fn.detach. You'll use $.fn.remove when you want to permanently remove the selection from the page; while the method does return the removed element(s), those elements will not have their associated data and events attached to them if you return them to the page.

If you need the data and events to persist, you'll want to use $.fn.detach instead. Like$.fn.remove, it returns the selection, but it also maintains the data and events associated with the selection, so you can restore the selection to the page at a later time.

Note

The $.fn.detach method is extremely valuable if you are doing heavy manipulation to an element. In that case, it's beneficial to $.fn.detach the element from the page, work on it in your code, and then restore it to the page when you're done. This saves you from expensive "DOM touches" while maintaining the element's data and events.

If you want to leave the element on the page but simply want to remove its contents, you can use$.fn.empty to dispose of the element's inner HTML.

Creating New Elements

jQuery offers a trivial and elegant way to create new elements using the same $() method you use to make selections.

Example 3.29. Creating new elements

$('<p>This is a new paragraph</p>');
$('<li class="new">new list item</li>');

Example 3.30. Creating a new element with an attribute object

$('<a/>', { 
    html : 'This is a <strong>new</strong> link',
    'class' : 'new',
    href : 'foo.html'
});

Note that in the attributes object we included as the second argument, the property name class is quoted, while the property names text and href are not. Property names generally do not need to be quoted unless they are reserved words (as class is in this case).

When you create a new element, it is not immediately added to the page. There are several ways to add an element to the page once it's been created.

Example 3.31. Getting a new element on to the page

var $myNewElement = $('<p>New element</p>');
$myNewElement.appendTo('#content');

$myNewElement.insertAfter('ul:last'); // this will remove the p from #content!
$('ul').last().after($myNewElement.clone());  // clone the p so now we have 2

Strictly speaking, you don't have to store the created element in a variable -- you could just call the method to add the element to the page directly after the $(). However, most of the time you will want a reference to the element you added, so you don't need to select it later.

You can even create an element as you're adding it to the page, but note that in this case you don't get a reference to the newly created element.

Example 3.32. Creating and adding an element to the page at the same time

$('ul').append('<li>list item</li>');

Note

The syntax for adding new elements to the page is so easy, it's tempting to forget that there's a huge performance cost for adding to the DOM repeatedly. If you are adding many elements to the same container, you'll want to concatenate all the html into a single string, and then append that string to the container instead of appending the elements one at a time. You can use an array to gather all the pieces together, then join them into a single string for appending.

var myItems = [], $myList = $('#myList');

for (var i=0; i<100; i++) {
    myItems.push('<li>item ' + i + '</li>');
}

$myList.append(myItems.join(''));

Manipulating Attributes

jQuery's attribute manipulation capabilities are extensive. Basic changes are simple, but the $.fn.attr method also allows for more complex manipulations.

Example 3.33. Manipulating a single attribute

$('#myDiv a:first').attr('href', 'newDestination.html');

Example 3.34. Manipulating multiple attributes

$('#myDiv a:first').attr({
    href : 'newDestination.html',
    rel : 'super-special'
});

Example 3.35. Using a function to determine an attribute's new value

$('#myDiv a:first').attr({
    rel : 'super-special',
    href : function() { 
        return '/new/' + $(this).attr('href');
    }
});

$('#myDiv a:first').attr('href', function() {
    return '/new/' + $(this).attr('href');
});

Copyright Rebecca Murphey, released under the Creative Commons Attribution-Share Alike 3.0 United States license.