You cannot safely manipulate a page until the document is “ready.” jQuery detects this state of readiness for you; code included inside $(document).ready()
will only run once the page is ready for JavaScript code to execute.
A $(document).ready() block
$(document).ready(function() { console.log('ready!'); });
There is a shorthand for $(document).ready()
that you will sometimes see; however, I recommend against using it if you are writing code that people who aren't experienced with jQuery may see.
Shorthand for $(document).ready()
$(function() { console.log('ready!'); });
You can also pass a named function to $(document).ready()
instead of passing an anonymous function.
Passing a named function instead of an anonymous function
function readyFn() { // code to run when the document is ready } $(document).ready(readyFn);
The most basic concept of jQuery is to “select some elements and do something with them.” jQuery supports most CSS3 selectors, as well as some non-standard selectors. For a complete selector reference, visit http://api.jquery.com/category/selectors/.
Following are a few examples of common selection techniques.
Selecting elements by ID
$('#myId'); // note IDs must be unique per page
Selecting elements by class name
$('div.myClass'); // performance improves if you specify element type
Selecting elements by attribute
$('input[name=first_name]'); // beware, this can be very slow
Selecting elements by compound CSS selector
$('#contents ul.people li');
Pseudo-selectors
$('a.external:first'); $('tr:odd'); $('#myForm :input'); // select all input-like elements in a form $('div:visible'); $('div:gt(2)'); // all except the first three divs $('div:animated'); // all currently animated divs
When you use the :visible
and :hidden
pseudo-selectors, jQuery tests the actual visibility of the element, not its CSS visibility or display — that is, it looks to see if the element's physical height and width on the page are both greater than zero. However, this test doesn't work with <tr>
elements; in this case, jQuery does check the CSS display
property, and considers an element hidden if its display
property is set to none
. Elements that have not been added to the DOM will always be considered hidden, even if the CSS that would affect them would render them visible. (See the Manipulation section later in this chapter to learn how to create and add elements to the DOM.)
For reference, here is the code jQuery uses to determine whether an element is visible or hidden, with comments added for clarity:
jQuery.expr.filters.hidden = function( elem ) { var width = elem.offsetWidth, height = elem.offsetHeight, skip = elem.nodeName.toLowerCase() === "tr"; // does the element have 0 height, 0 width, // and it's not a <tr>? return width === 0 && height === 0 && !skip ? // then it must be hidden true : // but if it has width and height // and it's not a <tr> width > 0 && height > 0 && !skip ? // then it must be visible false : // if we get here, the element has width // and height, but it's also a <tr>, // so check its display property to // decide whether it's hidden jQuery.curCSS(elem, "display") === "none"; }; jQuery.expr.filters.visible = function( elem ) { return !jQuery.expr.filters.hidden( elem ); };
Once you've made a selection, you'll often want to know whether you have anything to work with. You may be inclined to try something like:
if ($('div.foo')) { ... }
This won't work. When you make a selection using $()
, an object is always returned, and objects always evaluate to true
. Even if your selection doesn't contain any elements, the code inside the if
statement will still run.
Instead, you need to test the selection's length property, which tells you how many elements were selected. If the answer is 0, the length property will evaluate to false when used as a boolean value.
Testing whether a selection contains elements
if ($('div.foo').length) { ... }
Every time you make a selection, a lot of code runs, and jQuery doesn't do caching of selections for you. If you've made a selection that you might need to make again, you should save the selection in a variable rather than making the selection repeatedly.
Storing selections in a variable
var $divs = $('div');
In “Storing selections in a variable”, the variable name begins with a dollar sign. Unlike in other languages, there's nothing special about the dollar sign in JavaScript — it's just another character. We use it here to indicate that the variable contains a jQuery object. This practice — a sort of Hungarian notation — is merely convention, and is not mandatory.
Once you've stored your selection, you can call jQuery methods on the variable you stored it in just like you would have called them on the original selection.
A selection only fetches the elements that are on the page when you make the selection. If you add elements to the page later, you'll have to repeat the selection or otherwise add them to the selection stored in the variable. Stored selections don't magically update when the DOM changes.
Sometimes you have a selection that contains more than what you're after; in this case, you may want to refine your selection. jQuery offers several methods for zeroing in on exactly what you're after.
Refining selections
$('div.foo').has('p'); // div.foo elements that contain <p>'s $('h1').not('.bar'); // h1 elements that don't have a class of bar $('ul li').filter('.current'); // unordered list items with class of current $('ul li').first(); // just the first unordered list item $('ul li').eq(5); // the sixth
jQuery offers several pseudo-selectors that help you find elements in your forms; these are especially helpful because it can be difficult to distinguish between form elements based on their state or type using standard CSS selectors.
Selects <button>
elements and elements with type="button"
Selects inputs with type="checkbox"
Selects checked inputs
Selects disabled form elements
Selects enabled form elements
Selects inputs with type="file"
Selects inputs with type="image"
Selects <input>
, <textarea>
, and <select>
elements
Selects inputs with type="password"
Selects inputs with type="radio"
Selects inputs with type="reset"
Selects options that are selected
Selects inputs with type="submit"
Selects inputs with type="text"
Using form-related pseduo-selectors
$('#myForm :input'); // get all elements that accept input
Once you have a selection, you can call methods on the selection. Methods generally come in two different flavors: getters and setters. Getters return a property of the first selected element; setters set a property on all selected elements.
If you call a method on a selection and that method returns a jQuery object, you can continue to call jQuery methods on the object without pausing for a semicolon.
Chaining
$('#content').find('h3').eq(2).html('new text for the third h3!');
If you are writing a chain that includes several steps, you (and the person who comes after you) may find your code more readable if you break the chain over several lines.
Formatting chained code
$('#content') .find('h3') .eq(2) .html('new text for the third h3!');
If you change your selection in the midst of a chain, jQuery provides the $.fn.end
method to get you back to your original selection.
Restoring your original selection using $.fn.end
$('#content') .find('h3') .eq(2) .html('new text for the third h3!') .end() // restores the selection to all h3's in #content .eq(0) .html('new text for the first h3!');
Chaining is extraordinarily powerful, and it's a feature that many libraries have adapted since it was made popular by jQuery. However, it must be used with care. Extensive chaining can make code extremely difficult to modify or debug. There is no hard-and-fast rule to how long a chain should be — just know that it is easy to get carried away.
jQuery “overloads” its methods, so the method used to set a value generally has the same name as the method used to get a value. When a method is used to set a value, it is called a setter. When a method is used to get (or read) a value, it is called a getter. Setters affect all elements in a selection; getters get the requested value only for the first element in the selection.
The $.fn.html
method used as a setter
$('h1').html('hello world');
The html method used as a getter
$('h1').html();
Setters return a jQuery object, allowing you to continue to call jQuery methods on your selection; getters return whatever they were asked to get, meaning you cannot continue to call jQuery methods on the value returned by the getter.
jQuery includes a handy way to get and set CSS properties of elements.
CSS properties that normally include a hyphen need to be camel cased in JavaScript. For example, the CSS property font-size
is expressed as fontSize
when used as a property name in JavaScript. This does not apply, however, when passing the name of a CSS property to the $.fn.css
method as a string — in that case, either the camel cased or hyphenated form will work.
Getting CSS properties
$('h1').css('fontSize'); // returns a string such as "19px" $('h1').css('font-size'); // also works
Setting CSS properties
$('h1').css('fontSize', '100px'); // setting an individual property $('h1').css({ 'fontSize' : '100px', 'color' : 'red' }); // setting multiple properties
Note the style of the argument we use on the second line — it is an object that contains multiple properties. This is a common way to pass multiple arguments to a function, and many jQuery setter methods accept objects to set mulitple values at once.
As a getter, the $.fn.css
method is valuable; however, it should generally be avoided as a setter in production-ready code, because you don't want presentational information in your JavaScript. Instead, write CSS rules for classes that describe the various visual states, and then simply change the class on the element you want to affect.
Working with classes
var $h1 = $('h1'); $h1.addClass('big'); $h1.removeClass('big'); $h1.toggleClass('big'); if ($h1.hasClass('big')) { ... }
Classes can also be useful for storing state information about an element, such as indicating that an element is selected.
jQuery offers a variety of methods for obtaining and modifying dimension and position information about an element.
The code in “Basic dimensions methods”, is just a very brief overview of the dimensions functionality in jQuery; for complete details about jQuery dimension methods, visit http://api.jquery.com/category/dimensions/.
Basic dimensions methods
$('h1').width('50px'); // sets the width of all H1 elements $('h1').width(); // gets the width of the first H1 $('h1').height('50px'); // sets the height of all H1 elements $('h1').height(); // gets the height of the first H1 $('h1').position(); // returns an object containing position // information for the first H1 relative to // its "offset (positioned) parent"
An element's attributes can contain useful information for your application, so it's important to be able to get and set them.
The $.fn.attr
method acts as both a getter and a setter. As with the $.fn.css
method, $.fn.attr
as a setter can accept either a key and a value, or an object containing one or more key/value pairs.
Setting attributes
$('a').attr('href', 'allMyHrefsAreTheSameNow.html'); $('a').attr({ 'title' : 'all titles are the same too!', 'href' : 'somethingNew.html' });
This time, we broke the object up into multiple lines. Remember, whitespace doesn't matter in JavaScript, so you should feel free to use it liberally to make your code more legible! You can use a minification tool later to strip out unnecessary whitespace for production.
Getting attributes
$('a').attr('href'); // returns the href for the first a element in the document
Once you have a jQuery selection, you can find other elements using your selection as a starting point.
For complete documentation of jQuery traversal methods, visit http://api.jquery.com/category/traversing/.
Be cautious with traversing long distances in your documents — complex traversal makes it imperative that your document's structure remain the same, something that's difficult to guarantee even if you're the one creating the whole application from server to client. One- or two-step traversal is fine, but you generally want to avoid traversals that take you from one container to another.
Moving around the DOM using traversal methods
$('h1').next('p'); $('div:visible').parent(); $('input[name=first_name]').closest('form'); $('#myList').children(); $('li.selected').siblings();
You can also iterate over a selection using $.fn.each
. This method iterates over all of the elements in a selection, and runs a function for each one. The function receives the index of the current element and the DOM element itself as arguments. Inside the function, the DOM element is also available as this
by default.
Iterating over a selection
$('#myList li').each(function(idx, el) { console.log( 'Element ' + idx + 'has the following html: ' + $(el).html() ); });
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, visit http://api.jquery.com/category/manipulation/.
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.
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.
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.
Get or set the html contents.
Get or set the text contents; HTML will be stripped.
Get or set the value of the provided attribute.
Get or set the width in pixels of the first element in the selection as an integer.
Get or set the height in pixels of the first element in the selection as an integer.
Get an object with position information for the first element in the selection, relative to its first positioned ancestor. This is a getter only.
Get or set the value of form elements.
Changing the HTML of an element
$('#myDiv p:first') .html('New <strong>first</strong> paragraph!');
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.insertAfter
method 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.
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
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.
Making a copy of an element
// copy the first list item to the end of the list $('#myList li:first').clone().appendTo('#myList');
If you need to copy related data and events, be sure to pass true
as an argument to $.fn.clone
.
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.
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.
jQuery offers a trivial and elegant way to create new elements using the same $()
method you use to make selections.
Creating new elements
$('<p>This is a new paragraph</p>'); $('<li class="new">new list item</li>');
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.
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.
Creating and adding an element to the page at the same time
$('ul').append('<li>list item</li>');
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(''));
jQuery's attribute manipulation capabilities are extensive. Basic changes are simple, but the $.fn.attr method also allows for more complex manipulations. It can either set an explicit value, or set a value using the return value of a function. When the function syntax is used, the function receives two arguments: the zero-based index of the element whose attribute is being changed, and the current value of the attribute being changed.
Manipulating a single attribute
$('#myDiv a:first').attr('href', 'newDestination.html');
Manipulating multiple attributes
$('#myDiv a:first').attr({ href : 'newDestination.html', rel : 'super-special' });
Using a function to determine an attribute's new value
$('#myDiv a:first').attr({ rel : 'super-special', href : function(idx, href) { return '/new/' + href; } }); $('#myDiv a:first').attr('href', function(idx, href) { return '/new/' + href; });
Open the file /exercises/index.html
in your browser. Use the file /exercises/js/sandbox.js
or work in Firebug to accomplish the following:
Select all of the div elements that have a class of "module".
Come up with three selectors that you could use to get the third item in the #myList unordered list. Which is the best to use? Why?
Select the label for the search input using an attribute selector.
Figure out how many elements on the page are hidden (hint: .length).
Figure out how many image elements on the page have an alt attribute.
Select all of the odd table rows in the table body.
Open the file /exercises/index.html
in your browser. Use the file /exercises/js/sandbox.js
or work in Firebug to accomplish the following:
Select all of the image elements on the page; log each image's alt attribute.
Select the search input text box, then traverse up to the form and add a class to the form.
Select the list item inside #myList that has a class of "current" and remove that class from it; add a class of "current" to the next list item.
Select the select element inside #specials; traverse your way to the submit button.
Select the first list item in the #slideshow element; add the class "current" to it, and then add a class of "disabled" to its sibling elements.
Open the file /exercises/index.html
in your browser. Use the file /exercises/js/sandbox.js
or work in Firebug to accomplish the following:
Add five new list items to the end of the unordered list #myList. Hint:
for (var i = 0; i<5; i++) { ... }
Remove the odd list items
Add another h2 and another paragraph to the last div.module
Add another option to the select element; give the option the value "Wednesday"
Add a new div.module to the page after the last one; put a copy of one of the existing images inside of it.