understanding jquery

The jQuery library makes it easy to manipulate a page of HTML after it's displayed by the browser. It also provides tools that help you listen for a user to interact with your page, tools that help you create animations in your page, and tools that let you communicate with a server without reloading the page. We'll get to those in a bit. First, let's look at some jQuery basics, and at how we can use jQuery to perform its core functionality: getting some elements and doing something with them.

This guide assumes that you understand HTML and CSS Selectors. If you are not familiar with how you can use CSS selectors to target elements, you should spend some time getting up to speed before trying to work through this guide.

What’s $, anyway?

The jQuery library provides the jQuery function, which lets you select elements using CSS selectors.

 

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var listItems = jQuery( 'li' );

Of course, if you've seen any jQuery code, you're probably more accustomed to seeing something like this:

 

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var listItems = $( 'li' );

As discussed in the JavaScript Basics section, valid names in JavaScript can be pretty much anything, as long as they don't begin with a number and don't include a hyphen. So, the $ in the code above is just a shorter, more convenient name for the jQuery function; indeed, in the jQuery source code, you'll find this near the end:

 

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// Expose jQuery to the global object

window.jQuery = window.$ = jQuery;

When you call the $() function and pass a selector to it, you create a new jQuery object. Of course, in JavaScript, functions are objects too, so that means that $ (and jQuery, of course) has properties and methods, too. For example, you can refer to the $.support property for information on what the current browser environment supports, and you use the $.ajax method to make an AJAX request.

For the rest of this guide, we'll use $ instead of jQuery for the sake of brevity. Note that if your page contains more than one JavaScript library, then $ may be used by another library, which can cause jQuery not to work. If you experience this, you should consider using jQuery.noConflict before loading the other libraries.

$(document).ready()

Before you can safely use jQuery to do anything to your page, you need to ensure that the page is in a state where it's ready to be manipulated. With jQuery, we accomplish this by putting our code in a function, and then passing that function to $(document).ready(). As you can see here, the function we pass can just be an anonymous function.

 

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$( document ).ready(function() {

  console.log( 'ready!' );

});

This will run the function that we pass to .ready() once the document is ready. What's going on here? We're using $(document) to create a jQuery object from our page's document, and then calling the .ready() function on that object, passing it the function we want to execute.

Since this is something you'll find yourself doing a lot, there's a shorthand method for this if you prefer — the $() function does double duty as an alias for $(document).ready() if you pass it a function:

 

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$(function() {

  console.log( 'ready!' );

});

For the rest of this guide, we'll assume that the code we're looking at is enclosed in $(document).ready(function() { ... });, and we'll leave that part out for brevity.

Get some elements

The simplest thing we can do with jQuery is get some elements and do something with them. If you understand CSS selectors, getting some elements is very straightforward: you simply pass the appropriate selector to $().

 

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$( '#header' ); // select the element with an ID of 'header'

$( 'li' );      // select all list items on the page

$( 'ul li' );   // select list items that are in unordered lists

$( '.person' ); // select all elements with a class of 'person'

It's important to understand that any selection you make will only contain elements that existed in the page when you made the selection. That is, if you write var anchors = $( 'a' ); and then add another <a> element to your page later, then your anchors variable will not contain that new element.

Other ways to create a jQuery object

In addition to passing a simple selector to $(), you can create jQuery objects in a few other ways:

 

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// create a jQuery object from a DOM element

$( document.body.children[0] );

// create a jQuery object from a list of DOM elements

$( [ window, document ] );

// make a selection in the context of a DOM element

var firstBodyChild = document.body.children[0];

$( 'li', firstBodyChild );

// make a selection within a previous selection

var paragraph = $( 'p' );

$( 'a', paragraph );

Did my selection get anything?

Sometimes, you'll only want to do something when your selection matches some elements. Because the $() function always returns a jQuery object, and an object is always truthy, you'll need to test the contents of your selection to determine whether anything was found.

CAUTION BROKEN CODE

 

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if ( $( '#nonexistent' ) ) {

  // Wrong! This code will always run!

}

if ( $( '#nonexistent' ).length > 0 ) {

  // Correct! This code will only run if there's an element in your page

  // with an ID of 'nonexistent'

}

We can shorten our check even further if we remember that 0 is a falsy value:

 

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if ( $( '#nonexistent' ).length ) {

  // This code will only run if there's a matching element

}

Getting single elements from a selection

If you need to work with the raw DOM element from a selection, you need to access that element from the jQuery object. For example, if you wanted to access an <input> element's value property directly, you would want to work with the raw DOM element.

 

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var listItems = $( 'li' );

var rawListItem = listItems[0]; // or listItems.get( 0 )

var html = rawListItem.innerHTML;

Note that you cannot call jQuery methods on raw DOM elements. So, the following will not work:

CAUTION BROKEN CODE

 

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var listItems = $( 'li' );

var rawListItem = listItems[0];

var html = rawListItem.html();

// Object #<HTMLInputElement> has no method 'html'

If you need to work with a single element in a selection and you want to be able to use jQuery methods on that element, then you can get a new jQuery object containing a single element by using .eq() and passing the index of the element you're after.

 

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var listItems = $( 'li' );

var secondListItem = listItems.eq( 1 );

secondListItem.remove();

Creating new elements

The $ function has one last role: creating new elements. If you pass an HTML snippet to $(), it will create a new element in memory — that is, the element will be created, but it won't be placed on the page until you place it on the page.

 

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$( '<p>' ); // creates a new <p> element with no content

$( '<p>Hello!</p>' ); // creates a new <p> element with content

$( '<p class="greet">Hello!</p>' ); // creates a new <p> with content and class

You can also create an element by passing an object with information about how to create the element:

 

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$( '<p>', {

  html: 'Hello!',

  'class': 'greet'

});

Note that we must wrap the class property in quotation marks, as class is a reserved word in JavaScript, and failing to quote it will cause errors in some browsers. See the jQuery documentation for details on the various properties you can include in the object.

We'll look at how to place created elements into the document in the next chapter, which covers traversing and manipulating the document.

Working with selections

Once you've created a jQuery object that contains your selection, you probably want to do something with your selection. Before we do that, though, there are a few concepts that are key to understanding the jQuery way of doing things.

Testing a selection

We can determine whether a selection meets certain criteria using the .is() method. The most common way to use this method is to provide a selector as its sole argument. It returns true or false depending on whether the selection matches the selector:

 

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$( 'li' ).eq( 0 ).is( '.special' ); // false

$( 'li' ).eq( 1 ).is( '.special' ); // true

You can also pass to the .is() method a jQuery object, a raw DOM element, or even a function if you need to do a more complex test. See the documentation for more details.

Getters, setters, and implicit iteration

There are many methods you can call once you've made a selection. These methods generally fall into two categories: getters and setters. Getters retrieve a piece of information from the selection, and setters alter the selection in some way. In almost all cases, getters operate only on the first element in a selection (.text() is a notable exception); setters operate on all elements in a selection, using what's known as implicit iteration.

Implicit iteration means that jQuery automatically iterates over all the elements in a selection when you call a setter method on that selection. This means that, when you want to do something to all of the elements in a selection, you don't have to call a setter method on every item in your selection — you just call the method on the selection itself, and jQuery iterates over the elements for you.

Let's say that you want to change the HTML of all of the list items on your page. To do this, we'd use the .html() method, which will change the HTML of all of the selected list items.

 

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$( 'li' ).html( 'New HTML' );

You can also pass a function to jQuery's setter methods. This function's return value is used as the new value, and it receives two arguments: the index of the element in the selection, and the old value of the thing you're trying to change. This is useful when you need information about an element's current state in order to properly set the new state.

 

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$( 'li' ).html(function( index, oldHtml ) {

  return oldHtml + '!!!'

});

Explicit iteration

Sometimes, the task you're trying to accomplish won't fit neatly into one of jQuery's existing methods, and you'll need to explicitly iterate over a selection. The .each() method lets you do this. In the following code, we use it to add a <b> tag at the beginning of the list item, containing the index of the list item.

 

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$( 'li' ).each(function( index, elem ) {

  // this: the current, raw DOM element

  // index: the current element's index in the selection

  // elem: the current, raw DOM element (same as this)

  $( elem ).prepend( '<b>' + index + ': </b>' );

});

You'll notice that, inside the function that we pass to .each(), we have access to the current raw DOM element in two ways: as this and as elem. As discussed in the JavaScript Basics section, this is a special keyword in JavaScript, referring to the object that is the current context of the function.

In jQuery, this almost always refers to the raw DOM element on which the function is currently operating. So, in the case of .each(), it refers to the current element in the set of elements we're iterating over.

Chaining

One of the most lucrative parts of jQuery is the ability to "chain" methods together. This means that we can call a series of methods on a selection without having to repeat the selection or store the selection in a variable. We can even make new selections based on previous selections, all without breaking the chain.

 

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$( 'li' )

  .click(function() {

    $( this ).addClass( 'clicked' );

  })

  .find( 'span' )

    .attr( 'title', 'Hover over me' );

Chaining is possible because every setter method in jQuery returns the selection on which it was called. It's extraordinarily powerful, and it's a feature that many libraries have adopted. However, it must be used with care. Extensive chaining can make code extremely difficult to read, modify, and debug. There is no hard-and-fast rule on how long a chain should be, but even the simple chain above is probably worth refactoring for readability.

 

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var listItems = $( 'li' );

var spans = listItems.find( 'span' );

listItems

  .click(function() {

    $( this ).addClass( 'clicked' );

  });

spans.attr( 'title', 'Hover over me' );

Conclusion

We've gotten a great overview of how jQuery ticks; in the next section, we'll take a look at how to actually start accomplishing things with it!