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 the Selectors documentation on api.jquery.com.
link Selecting Elements by ID
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link Selecting Elements by Class Name
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link Selecting Elements by Attribute
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link Selecting Elements by Compound CSS Selector
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link Selecting Elements with a Comma-separated List of Selectors
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link Pseudo-Selectors
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Note: When using the :visible
and :hidden
pseudo-selectors, jQuery tests the actual visibility of the element, not its CSS visibility
or display
properties. jQuery 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 the case of <tr>
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 Manipulating Elements section to learn how to create and add elements to the DOM.
link Choosing Selectors
Choosing good selectors is one way to improve JavaScript's performance. Too much specificity can be a bad thing. A selector such as #myTable thead tr th.special
is overkill if a selector such as #myTable th.special
will get the job done.
link Does My Selection Contain Any Elements?
Once you've made a selection, you'll often want to know whether you have anything to work with. A common mistake is to use:
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This won't work. When a selection is made using $()
, an object is always returned, and objects always evaluate to true
. Even if the selection doesn't contain any elements, the code inside the if
statement will still run.
The best way to determine if there are any elements is 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:
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link Saving Selections
jQuery doesn't cache elements 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.
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Once the selection is stored in a variable, you can call jQuery methods on the variable just like you would have called them on the original selection.
A selection only fetches the elements that are on the page at the time the selection is made. If elements are added 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.
link Refining & Filtering Selections
Sometimes the selection contains more than what you're after. jQuery offers several methods for refining and filtering selections.
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link Selecting Form Elements
jQuery offers several pseudo-selectors that help find elements in 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.
link :checked
Not to be confused with :checkbox, :checked
targets checked checkboxes, but keep in mind that this selector works also for checked radio buttons, and <select>
elements (for <select>
elements only, use the :selected
selector):
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The :checked
pseudo-selector works when used with checkboxes, radio buttons and selects.
link :disabled
Using the :disabled
pseudo-selector targets any <input>
elements with the disabled
attribute:
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In order to get the best performance using :disabled
, first select elements with a standard jQuery selector, then use .filter( ":disabled" )
, or precede the pseudo-selector with a tag name or some other selector.
link :enabled
Basically the inverse of the :disabled pseudo-selector, the :enabled
pseudo-selector targets any elements that do not have a disabled
attribute:
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In order to get the best performance using :enabled
, first select elements with a standard jQuery selector, then use .filter( ":enabled" )
, or precede the pseudo-selector with a tag name or some other selector.
link :input
Using the :input
selector selects all <input>
, <textarea>
, <select>
, and <button>
elements:
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link :selected
Using the :selected
pseudo-selector targets any selected items in <option>
elements:
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In order to get the best performance using :selected
, first select elements with a standard jQuery selector, then use .filter( ":selected" )
, or precede the pseudo-selector with a tag name or some other selector.
link Selecting by type
jQuery provides pseudo selectors to select form-specific elements according to their type:
- :password
- :reset
- :radio
- :text
- :submit
- :checkbox
- :button
- :image
- :file
For all of these there are side notes about performance, so be sure to check out the API docs for more in-depth information.