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Reading and writing from the DOM

From: Foundations of Programming: Fundamentals

Video: Reading and writing from the DOM

JavaScript is a scripting language, not a general purpose programming language, and its world is the webpage that loaded it, not your operating system, not your hard drive. Instead, it has some great functions for grabbing and manipulating what's on the webpage. JavaScript is using something called the Document Object Model or the DOM to do this. Now, I can't talk much about the DOM without also talking a little bit about HTML and webpages in general. So while the next few minutes by itself is a little specialized to be something I'd call a general programming fundamental, well, that's not really about this.

Reading and writing from the DOM

JavaScript is a scripting language, not a general purpose programming language, and its world is the webpage that loaded it, not your operating system, not your hard drive. Instead, it has some great functions for grabbing and manipulating what's on the webpage. JavaScript is using something called the Document Object Model or the DOM to do this. Now, I can't talk much about the DOM without also talking a little bit about HTML and webpages in general. So while the next few minutes by itself is a little specialized to be something I'd call a general programming fundamental, well, that's not really about this.

It's the bigger picture here that's important. And the idea which is fundamental is that the languages you use become specialized to the world that they work in. So if you work designing mobile applications, well you'll end up being able to write code to deal with what happens if the phone rings while your program is running. But you would not find that same ability when you're writing desktop applications. Instead, you might be able to write code to deal with the printer or the USB port because that would be your world.

And if you work in JavaScript, you'll find yourself being able to write code to deal with the webpage. You see, we've been focused on using JavaScript so far to illustrate the basics of programming, things like conditions, iterations, arrays, and using these annoying alert boxes. But we know JavaScript doesn't run with alert boxes all the time. Real-world JavaScript is all about reaching from our script into the webpage that we have, and causing that page to reach back into our script.

And we do that by understanding this thing called the DOM, the Document Object Model. Now, this is the term that's a little odd to understand the first time you hear it because it's kind of vague. People ask me, well, what is the DOM? Is it a language, is it part of JavaScript? What is it? Well, DOM is actually a pretty simple idea. And the best way to understand it is really to pull it apart into its three different words. Document, Object, Model. So we're going to take it apart piece by piece. We'll start with the first one. What do we mean by document? Well, if I'm in Microsoft Word, document might mean one thing, whatever I'm working on there, but we're not.

We're on a webpage. For us, document simply means the page. Our JavaScript was loaded by a webpage and that page is the document as far as the script is concerned. But this document can be represented in different ways. It already is. We know this. We know that our page is written in HTML. So between the two here, which one will be the document? Well, both of them. One is the browser view, and one is the HTML code. It's perfectly allowable to have the same document with a different representation.

It's the same idea here, but JavaScript perceives the same webpage a third way, Same document with a different representation. So if the current page is all we mean by Document, what's the Object part of Document Object Model? Well, Object here just means the different elements, the components, the pieces of this document. If I was to talk to a user, I could say look at the headline on this page, look at the bullet point list on this page, look at the whole page, or even look at the third letter in the second word.

But if you're a web developer, I could use a different perspective. I could say look at the HTML, look at the h1 that represents the headline, look at the unordered list part, look at the whole document. All these things are objects. And yes, sometimes one object contains other object. That's okay. We'll have a way in JavaScript to get to every single piece, every object at whatever level that makes sense, from being able to grab the entire document to being able to grab the smallest piece.

That leads us to the idea of the model. The model simply means what do we call these individual pieces and how do we describe the relationships between them. The model here is like a business model or a data model. It's trying to represent something complex by making it abstract, making it simple. It's simply a set of terms that we can agree on. Do we call these individual pieces elements or nodes or doohickeys? What do we call them? We all have to agree. What we end up with is that the DOM in JavaScript is an agreed-upon set of terms.

That's our model. That describes how to interact with the pieces of a webpage. And that's a pretty long phrase, so we call it the Document Object Model. It is not a language, it's an idea. But it's an idea that JavaScript agrees on and that the web browsers agree on. So it's available there. Using the DOM, it lets us describe and interact with any webpage. Not any one specific page, but any webpage. When I know the basic concepts of the DOM, I could write some JavaScript that would say get the title text or get the second paragraph or get the third link in the menu and make it invisible, change the background color, get all the elements in the list, find an image and move it 40 pixels to the right, and a lot more besides.

And that's why if we're working in JavaScript we'd need to know the DOM. It is the way to reach into the page from our script and the way our page can reach into our script. Let me show you just one quick example. In my folder here, I have the same old couple of files. I'm going to open up this HTML page first in the browser. It's a little different, not very from what I've been looking at so far. If I open that up in the text editor, I can see that one thing I've added here, and I'm looking at both different views, is this.

I've added a headline. In the web browser, it shows up as this big bold Interesting Headline. In the HTML view, it's being represented by this h1 tag, and I've given it an ID of mainHeading. This is really like giving it its own unique label. Okay, so what's the point of this? Well, part of the idea of the DOM is that understand what it means to be a named element on a page. I can grab this headline if I know this name. So I'm going to leave the HTML here and go into my JavaScript.

What I can do is create a new variable. I'll call it headline and I want to grab that part of the webpage. The way that I do it is using the Document Object Model. I will say well, it's part of the document. Document is an object that actually exists anywhere in my JavaScript. Then I'm going to say that I want to get a particular element. There's a built-in function here called getElementById. It's one of the most popular functions in the whole of JavaScript.

I give it one piece of information, which is, what did I call it? Well, I called it mainHeading. And as soon as this line of code runs, I'll have this handle on that particular piece of the page. Then I can use this handle, this variable name, to say well, I'm going to reach inside of it, and set its own contents in our HTML to something new, "Wow, a new headline!" Okay, not the most exciting thing in the world. But if I save this, I jump back to my folder, and I run this page again, no alert box this time, but I can see that I'm immediately reaching into that headline, and actually changing the value of it.

It's not changing my actual HTML. If I look at the HTML file, it still says Interesting Headline. But what's happening is the HTML was loading, it's bringing in our JavaScript, and immediately our JavaScript is reaching back and changing the contents of this headline tag. And sure, while this by itself might not set your world on fire, if we can affect the headline, we can affect anything. We can change colors, we can change text. We are creating the contents of this page through our JavaScript.

All programming languages have an area in which they become specialized based on the world that they're working in. When you're a JavaScript developer, knowing the DOM and learning the DOM, being able to navigate through the page, is the single most important skill for being proficient in that language.

Show transcript

This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 88126 viewers

Simon Allardice
Author

 
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  1. 4m 15s
    1. Welcome
      1m 17s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 48s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 9s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 54s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 56s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 27s
  7. 13m 31s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 39s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 58s
    1. Working with arrays
      5m 46s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 25s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 2s
  13. 14m 16s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 28s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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