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Traversing documents

From: jQuery Essential Training

Video: Traversing documents

If you have done any web development in the past, then you probably know by now that traversing across the information returned from a document is something that's a very common operation and jQuery provides a lot of functions that make this process easier. Now, I have listed some of the functions that jQuery makes available to help with this process below. This is not an exhaustive list. Some of the functions in this area in jQuery are fairly advanced. So, I have chosen the ones that are most commonly used and you'll run into again and again. So, let's go over these. The first one that I want to go over is the size function and length property. They both do the same thing. When you execute a jQuery expression, and you get a result set back, this function or property, whichever one you prefer to use, will tell you the number of elements that are in the jQuery result set and you can use that information in conjunction with other jQuery functions to iterate over these items or do some other operations. So, we can also use this property or the function to test to see if the result of a jQuery expression came back with any elements at all, because sometimes you will need to know that information when you are doing certain things and we will see an example of that later on in the course. The next two functions, get and index, that returns an array of all the matched elements as DOM elements. Now remember in jQuery, the result set that come back from a jQuery selection is not an array of DOM elements. Those DOM elements have been wrapped inside jQuery objects, which have a lot of functionality built on top of them. So, these functions are pretty useful if you need to operate on the DOM elements themselves instead of using the built-in functions that jQuery gets you. So, the get function returns an array of all the elements and the result is set as DOM elements or you can use the version that takes the index parameter and that you will get you a single element at that index as a DOM element. And the last two functions, find and each, are also pretty powerful functions. The find function can be used to search inside a matched element for other elements that match the expression. You can imagine searching inside a div for images, for example. And then the last function, the each function, is also very powerful. The each function allows you to loop over the contents of every element in the result set of a jQuery selection and execute a function against that element. And again, this is a very powerful feature of jQuery. You will come across the need for this again and again. In fact, later on in the course, we will use the each function to solve a particular problem. So now that we have seen some of these functions and properties, let's jump over to the code and exercise them. All right, so here in the exercise_ files folder, I am going to open up the traversing_start file. You can view the finished version if you want or you can follow along with me in the start one. This is the document we have been using and it's got the paragraphs and the list items. So, let's begin by exercising some of the properties and functions we have learned. So, the first thing I am going to do is use the length property to inspect the result set of a jQuery selector. So, actually we are going to display it in an alert and I am going to display the string that says There are + and then I am going to execute my jQuery function and I am going to look for all the paragraphs tags and we can use the length property + and then say elements. So, this should now show me in an alert, how many paragraph elements there are. So, I am going to save and go to the browser. Okay, let's bring this up in the browser and refresh and you can see that sure enough there are four

elements, right? One, two, three, four, that's correct. All right, so let's go back to the code, just to show you you can use the size function as well. So, I am going to save and refresh and you can see that the same thing happened. Okay, let's go back to the code. This time let's use the get functions to retrieve the DOM elements. So, I am going to write var elems =. This time I am going to look for all the list items and I am going to write get, okay. So, that's going to get me array of all the list items as DOM elements. And I am going to write alert, There are " + elems.length; and this is the built-in JavaScript array length operator there. Just to avoid confusion, there is our li tags. So, that should now produce pretty much the same result as the paragraph example only now using the DOM instead. So, let's save and now we are going to refresh and sure enough there is four list item tags. So, let's go back to the code again. This time, let's use just the individual index operator on the get function. So this time I am going to write alert and I am going to alert li.get at 0. So, this will come back with an alert showing me what kind of object is at index 0. We will save and we are going to refresh and you can see that sure enough, there is an object and it's an HTML li element and that's correct, that's guy right there. It is, in fact, a list item and this is a DOM element object right here. So, that appears to be working. So, if I wanted to work on that li tag using the DOM, I can now do that. All right, let's go back to the code. Okay. Now, let's get a little more fancy. Let's use the find function. So we are going to use the find function to find list items that have a class b inside of the ul tag. And you can accomplish this using selector expressions, but this is just for the sake of illustration. There are much more powerful ways to use the find. I am just going to use this example to show you what it does. So, I am going to select the ul tag and then I am going to type find and I am going to say find list items that have the b class and I am going to use my same CSS trick from earlier. I am going to set the border to 3 pixel, solid red, so you can see it. So, what should happen is all the list items that have b applied should have a border around them. Okay, so we are going to save and we will refresh this in the browser and sure enough, it worked. So, the find function found those list items. One more example, let's go back into the code. All right, now are going to use the each function. And this is where the things get really interesting. What I am going to do is use the each function to apply progressively increasing values to the style applied to the paragraph elements. So, I am going to make two variables. I am going to make a variable called leftmargin and that's going to start at zero and I am going to make another variable called border and that's going to start as three. So, now I am going to write my jQuery expression to find all the paragraphs tags. I am going to say .each. So, for each paragraph tag, I want to execute a function. And what this function is going to do is remember, inside this function for each one of these functions that's going to be called that this keyword is going to refer to each one of the paragraphs as it comes through. So, for each paragraph, I am going to set the CSS property for the border to be the value of the border variable + pixels and then solid red. So, the border is going to get bigger each time and then I am going to say (this).css. Only this time I am going to set the CSS for the margin-left of the paragraph to be the leftmargin. Then I am going to say border += 2, so the border is going to get thicker by 2 pixels each time and the leftmargin is going to get bigger by 10 each time. So, for each paragraph, these values are going to get larger and we can see the visual result of that when we refresh the page. So, save and go back to the browser. So, now when I refresh, you are going to see each one of these paragraphs should become increasingly indented to the right and they should each have a red border that gets progressively thicker and you can see that that's exactly what happened. That's how you use jQuery to traverse across the elements in a web page.

Traversing documents

If you have done any web development in the past, then you probably know by now that traversing across the information returned from a document is something that's a very common operation and jQuery provides a lot of functions that make this process easier. Now, I have listed some of the functions that jQuery makes available to help with this process below. This is not an exhaustive list. Some of the functions in this area in jQuery are fairly advanced. So, I have chosen the ones that are most commonly used and you'll run into again and again. So, let's go over these. The first one that I want to go over is the size function and length property. They both do the same thing. When you execute a jQuery expression, and you get a result set back, this function or property, whichever one you prefer to use, will tell you the number of elements that are in the jQuery result set and you can use that information in conjunction with other jQuery functions to iterate over these items or do some other operations. So, we can also use this property or the function to test to see if the result of a jQuery expression came back with any elements at all, because sometimes you will need to know that information when you are doing certain things and we will see an example of that later on in the course. The next two functions, get and index, that returns an array of all the matched elements as DOM elements. Now remember in jQuery, the result set that come back from a jQuery selection is not an array of DOM elements. Those DOM elements have been wrapped inside jQuery objects, which have a lot of functionality built on top of them. So, these functions are pretty useful if you need to operate on the DOM elements themselves instead of using the built-in functions that jQuery gets you. So, the get function returns an array of all the elements and the result is set as DOM elements or you can use the version that takes the index parameter and that you will get you a single element at that index as a DOM element. And the last two functions, find and each, are also pretty powerful functions. The find function can be used to search inside a matched element for other elements that match the expression. You can imagine searching inside a div for images, for example. And then the last function, the each function, is also very powerful. The each function allows you to loop over the contents of every element in the result set of a jQuery selection and execute a function against that element. And again, this is a very powerful feature of jQuery. You will come across the need for this again and again. In fact, later on in the course, we will use the each function to solve a particular problem. So now that we have seen some of these functions and properties, let's jump over to the code and exercise them. All right, so here in the exercise_ files folder, I am going to open up the traversing_start file. You can view the finished version if you want or you can follow along with me in the start one. This is the document we have been using and it's got the paragraphs and the list items. So, let's begin by exercising some of the properties and functions we have learned. So, the first thing I am going to do is use the length property to inspect the result set of a jQuery selector. So, actually we are going to display it in an alert and I am going to display the string that says There are + and then I am going to execute my jQuery function and I am going to look for all the paragraphs tags and we can use the length property + and then say elements. So, this should now show me in an alert, how many paragraph elements there are. So, I am going to save and go to the browser. Okay, let's bring this up in the browser and refresh and you can see that sure enough there are four

elements, right? One, two, three, four, that's correct. All right, so let's go back to the code, just to show you you can use the size function as well. So, I am going to save and refresh and you can see that the same thing happened. Okay, let's go back to the code. This time let's use the get functions to retrieve the DOM elements. So, I am going to write var elems =. This time I am going to look for all the list items and I am going to write get, okay. So, that's going to get me array of all the list items as DOM elements. And I am going to write alert, There are " + elems.length; and this is the built-in JavaScript array length operator there. Just to avoid confusion, there is our li tags. So, that should now produce pretty much the same result as the paragraph example only now using the DOM instead. So, let's save and now we are going to refresh and sure enough there is four list item tags. So, let's go back to the code again. This time, let's use just the individual index operator on the get function. So this time I am going to write alert and I am going to alert li.get at 0. So, this will come back with an alert showing me what kind of object is at index 0. We will save and we are going to refresh and you can see that sure enough, there is an object and it's an HTML li element and that's correct, that's guy right there. It is, in fact, a list item and this is a DOM element object right here. So, that appears to be working. So, if I wanted to work on that li tag using the DOM, I can now do that. All right, let's go back to the code. Okay. Now, let's get a little more fancy. Let's use the find function. So we are going to use the find function to find list items that have a class b inside of the ul tag. And you can accomplish this using selector expressions, but this is just for the sake of illustration. There are much more powerful ways to use the find. I am just going to use this example to show you what it does. So, I am going to select the ul tag and then I am going to type find and I am going to say find list items that have the b class and I am going to use my same CSS trick from earlier. I am going to set the border to 3 pixel, solid red, so you can see it. So, what should happen is all the list items that have b applied should have a border around them. Okay, so we are going to save and we will refresh this in the browser and sure enough, it worked. So, the find function found those list items. One more example, let's go back into the code. All right, now are going to use the each function. And this is where the things get really interesting. What I am going to do is use the each function to apply progressively increasing values to the style applied to the paragraph elements. So, I am going to make two variables. I am going to make a variable called leftmargin and that's going to start at zero and I am going to make another variable called border and that's going to start as three. So, now I am going to write my jQuery expression to find all the paragraphs tags. I am going to say .each. So, for each paragraph tag, I want to execute a function. And what this function is going to do is remember, inside this function for each one of these functions that's going to be called that this keyword is going to refer to each one of the paragraphs as it comes through. So, for each paragraph, I am going to set the CSS property for the border to be the value of the border variable + pixels and then solid red. So, the border is going to get bigger each time and then I am going to say (this).css. Only this time I am going to set the CSS for the margin-left of the paragraph to be the leftmargin. Then I am going to say border += 2, so the border is going to get thicker by 2 pixels each time and the leftmargin is going to get bigger by 10 each time. So, for each paragraph, these values are going to get larger and we can see the visual result of that when we refresh the page. So, save and go back to the browser. So, now when I refresh, you are going to see each one of these paragraphs should become increasingly indented to the right and they should each have a red border that gets progressively thicker and you can see that that's exactly what happened. That's how you use jQuery to traverse across the elements in a web page.

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This video is part of

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jQuery Essential Training

49 video lessons · 97274 viewers

Joe Marini
Author

 
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  1. 2m 52s
    1. Welcome
      1m 9s
    2. Using the exercise files
      1m 43s
  2. 17m 37s
    1. What is jQuery?
      5m 19s
    2. Downloading and installing jQuery
      2m 20s
    3. Creating a simple jQuery-enabled page
      7m 12s
    4. Overview of features in jQuery
      2m 46s
  3. 59m 57s
    1. Overview of selectors and filters
      2m 9s
    2. Using basic jQuery selectors
      9m 6s
    3. Using basic jQuery filters
      8m 35s
    4. Using jQuery attribute filters
      6m 7s
    5. Child, visibility, and content filters
      9m 59s
    6. Form selectors and filters
      9m 3s
    7. Traversing documents
      9m 1s
    8. Understanding jQuery statement chaining
      1m 42s
    9. Practical example 1: Annotating page links
      4m 15s
  4. 47m 16s
    1. Creating, getting, and setting content
      5m 53s
    2. Manipulating attributes
      5m 43s
    3. Inserting content
      4m 57s
    4. Wrapping, replacing, and removing content
      5m 27s
    5. Working with CSS
      6m 19s
    6. Associating data with page elements
      9m 30s
    7. Practical example 2: Automatic TOC generator
      9m 27s
  5. 33m 6s
    1. Understanding the jQuery event handling features
      2m 4s
    2. Binding and unbinding events
      6m 23s
    3. Convenient event helper methods
      4m 40s
    4. Using the jQuery event object
      6m 21s
    5. Using miscellaneous event features
      4m 38s
    6. Practical example 3: Table striping and highlighting
      9m 0s
  6. 28m 45s
    1. Hiding and showing elements
      5m 23s
    2. Fading elements in and out
      4m 2s
    3. Sliding elements
      4m 3s
    4. Creating custom animations
      5m 58s
    5. Practical example 4: Image rotator
      9m 19s
  7. 25m 30s
    1. Introduction to jQuery UI
      3m 40s
    2. Exploring the jQuery UI widgets
      5m 24s
    3. Exploring the jQuery UI effects
      3m 58s
    4. Using the jQuery UI ThemeRoller
      4m 11s
    5. Downloading and installing jQuery UI
      8m 17s
  8. 47m 49s
    1. Overview of the sample web site
      3m 50s
    2. Using the accordion widget
      9m 14s
    3. Creating an image rotator
      10m 22s
    4. Building hover tooltips
      7m 26s
    5. Making an image selector
      9m 30s
    6. Using the Resizable effect
      7m 27s
  9. 30m 2s
    1. Working with Asynchronous JavaScript and XML (AJAX)
      10m 8s
    2. Using AJAX helpers
      4m 34s
    3. Understanding AJAX data types
      10m 14s
    4. Using global AJAX event handlers
      5m 6s
  10. 20s
    1. Goodbye
      20s

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