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Basic jQuery selectors are based on the familiar CSS syntax that you are probably already familiar with, and they work pretty much the same way that CSS does. I have listed here the basic CSS selectors in the table and you can see they correspond directly to their CSS counterparts. So, for example, if I pass a tag name to the jQuery selector, it will find me all elements that have that same tag name.
Similarly, if I pass in an identifier, jQuery will find the element that has that id. The same thing goes for class name. If I pass in a class to the jQuery selector, it will find all the elements that have a class attribute that have that class name. And just like CSS, you can get a little bit more fancy. You can look for tags that have a particular class name, or you can look for a tag that has a specific id and has a class name on it. And then, there is a wildcard character, the special character, the asterisk, basically means find all of the elements on the page.
And if that sometimes happens, you need to iterate over all of the elements in the page. So, let's take a look at how you would use jQuery versus using the plain browser DOM to get at information in a document. So, here I have a pretty fairly simple HTML file and it's been stripped down a bit for purposes of illustration. But you can see it has an unordered list in it and some list items and paragraph tags and so on. So, let's take a look at some comparisons of the DOM and jQuery.
If I want to get all the paragraph tags in the document, using the DOM, I would do something like this. I would use the document. getElementsByTagName and pass in the p, and that would get me all these paragraph tags. To do the same thing using jQuery, you can see all I have to do is call the jQuery object. That's the little $ sign right there. That's the global jQuery function, and I simply pass in the parameter p, and that will come back with what is called a matched set, or a wrap set it's sometimes called, or matched elements.
It comes back with a list of all the tags that match that particular parameter right there. Okay, suppose I wanted to get the tag that had the id "list1" and again using the DOM, I would use the standard DOM function getElementById, and I would give it the id that's this list right here. That's what the DOM would come back with. To do the same thing in jQuery, you can see I am just using standard CSS syntax, a little hash mark with the id in front of it, and that comes back with the element for that id.
Okay, let's get a little more fancy.
Suppose you want to get all the list item tags that have class a on them.
Well, to do that using the DOM, it's a little bit more complicated.
You would have to first use the getElementsByTagName function right to get all
And finally, let's imagine you wanted to have a situation where you wanted all
the tags regardless of what kind of tag they were that have a class b on them
but only if they are inside a
Again, using DOM this would be pretty complicated.
You have to get all the tags, see if they have the b class, see if they are
inside the ul, that kind of thing, but jQuery makes this really easy, again,
using pretty standard CSS syntax with the descendant inside the ancestor.
So, this basically says get me everything that has b class as long as
it's inside the ul.
That would come back with these list items right here. In addition to the basic selectors, there are some selectors that are a little bit more advanced. These are the hierarchy and combination selectors. So, these allow you to get a little bit more advanced in selecting page content. It basically allows you to select elements based on hierarchical relationships or a combination of criteria. So, the first two do just that, whereas in jQuery, usually you will see one selector passed in. If I passed in a comma-delimited list like p, div, image, it would find all of the elements that match everything in the list and for class 1 and class 2, this means find all the elements that have both class 1 and class 2 on them.
And they both have to be there. The next couple of examples illustrate hierarchical relationships. So, if you wanted to find all the child elements that are direct children of a certain parent, you would use this expression right here, whereas if all you cared about was finding a tag that appeared anywhere underneath a given ancestor, you would use this form, with the ancestor and then a space, and then whatever you are interested in here. In the next two, you have to deal with siblings. So, the next operator finds whatever element is next to this element, right, the previous element, and the siblings element says hey! Get me all the sibling elements that come after the previous element right here, and match the selector that I am passing in for siblings.
Okay, so let's take a look at some of these in action. We will switch over to code. Okay, so if you look inside your exercise files, I have got a file right here named Basicselector_start and I am going to start off there. So I am going to open this up in my editor and a couple of things we are going to do. So, you see that this is pretty much the document we are working with. Very similar to the one in the slides. Got an unordered list that's in paragraphs. Let's go back to the source. So, what I am going to do is write a function that executes when the page loads and I am going to do that on the ("document").ready event.
Okay, so here is what we are going to do. .ready. That's a function.
Let's try a couple of examples.
So, I will say get me the tags.
tags.Now, don't pay attention too much to this. What I am about to do. I want you to focus on the selectors right here. We will get to this stuff later. I just need to make the result visible, so you can see them on the screen. So, just bear with me while I do this. I am just going to make the CSS border a 3 pixel solid red line, so that we can see everything. What this is going to do is select all the paragraph tags. So, let's try that out in the browser.
And as you can see, all the paragraph tags are now inside bold red outlines. Let's go back to the code and try something else. Okay, let's do the same thing, only this time, we are going to do it for everything that has a class of a. So you can see that these list items, that paragraph, those should all be inside red borders. Let's try this. I am going to refresh, and you can see sure enough, that's the case. All right, now let's go back and try the id selector, back to the code. Okay, so now I am going to put in list1 and that's this unordered list right here.
So, now the list itself should have a border around it. Okay, let's refresh and sure enough that works. And then finally, let's one more. I am going to try all paragraphs that have class b assigned. All right, let's refresh and you can see that works just fine. Okay, so let's try some hierarchical ones now. Here we are in the system again and I am going to do the same thing. There is a file here called HierCombo. I am going to open that in my editor. Again, if you follow along with me, we are going to try some hierarchical relationships now.
What I am going to do first is try get me paragraphs and list items that have b applied, and apply that CSS trick, so we can see it. Okay, and let's bring this up in the browser. Okay, and you can see that that works, all the paragraphs and everything with the b. All right, let's try the descendant operator. And the descendant operator basically will ask for things inside an unordered list, and list items with an a on them.
I will just try that. Okay, we will refresh, and you can see that that worked. All right, now let's try the next operator. So, we are going to say get me the paragraph that's next to the unordered list. Refresh, and you can see that the first paragraph that was next to the list is now bordered. Okay, let's try the siblings one. For this one, I am going to say, let's see. How about whatever it is, let me get the list1 and get me the paragraph siblings of list1.
So, all these paragraphs are siblings of list1. Let's try that and sure enough it works. Okay, so now you have seen how to use basic jQuery selectors. Let's move on now to Filters.
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