webdev-basics/5-js-in-html-and-dom.md

JS in HTML & DOM

Linking JS to HTML

Motivation

During this course, we'll mostly focus on front-end development with React

However, it helps to understand some of the basics on how to interact with web pages using pure JavaScript

We won't dive deep into making complex websites with pure JavaScript without a framework, but we'll glance at some of the basics

• Javascript in an HTML file is written within the </html>

  Exercise 1: HTML + JS hello world

  Add some functionality to your HTML page.

  Create a function that prints "hello world" . Call this function in your script!

  Events

  • Creating a function and then running it when loading the page doesn't really add _interactivity _ to a website
    • Luckily, JavaScript functions can be bound to various events
  • The fastest way: define the function inside the

    JS has quite large amounts of built in events for different kind of user interactions.

    A list of these can be found in:

    https://developer.mozilla.org/en-US/docs/Web/Events

    and

    https://www.w3schools.com/jsref/dom_obj_event.asp

    Forms and JS example

    Username:

    Password:

    Login

    const loginForm = document.querySelector("#login");

    loginForm.addEventListener("submit", (e) => {

    e.preventDefault();

    const username = document.querySelector("#username");

    const password = document.querySelector("#password");

    console.log("username: ", username.value);

    console.log("password: ", password.value);

    });

    Event listener fires when form is submitted. The script selects username and password inputs and logs the values

    Exercise 2: Counter

    Add a button to your page. Add an event handler function to the button that uses the window.alert method to tell the user how many times they've clicked on the button so far.

    See https://developer.mozilla.org/en-US/docs/Web/API/Window/alert

    For example, on first press the user should see a message box saying "Clicked 1 time(s)", on second press it should say "Clicked 2 time(s)", and so on.

    Note: _ _ In real life, avoid using window.alert as it's generally not great for the user experience. Here it's fine for educational purposes.

    Exercise 3: Color change click!

    Create an HTML element whose color changes when it's clicked.

    First, use the fast "onclick" version

    Then, convert it into using addEventListener

    _Extra: _ Change the color of another element, not the one that was clicked!

    _Extra 2: _ Change the color into a new random color every time the button is clicked!

    Strict mode

    • Strict mode was introduced in ES5
    • Strict mode makes a few changes to normal JS semantics
      • Prohibits some JS syntax
      • Creates error logs for silent JS errors (that do not show up in the console)
      • Allows optimization of JS code
      • Strict mode is on automatically when using JavaScript modules i.e. using import and export

    Converting your JS files to strict mode

    • Make the first line of your code 'use strict'
    • It prevents you from creating code that is considered bad practice:
      • Declaring variable without a keyword (let, const, (or var))
      • Using the "with" statement (we won't get into that, don't use it)
      • Using "delete" on a variable
      • Declaring functions in blocks:
      • and so on…
      • https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

    if (q===w) function f() {}

    Window

    window is an object that contains a set of default variables and functions that you can use within a browser

    window.alert(), window.document, window.console.log(), window....

    As you can see, most of these functions look familiar. That's because you can access them also without window.

    In some sense, window keyword is just a list of global variables and functions.

    Be careful with window!

    Basically...

    window.console.log("asd") --> console.log("asd")

    window.alert("asd") --> alert("asd")

    If you give a variable or function a name that is already used within the "window" object by default, that variable/function gets overwritten!

    Separating scripts

    Separating JS from HTML completely

    Instead of including JS in the HTML file, we can separate it to another .js file

    Then, we can refer to the new JS file in our HTML file:

    There can be more than one of these links, e.g.,

    Note that the order of the script links matters!

    Multiple js files

    • If we have multiple JS files, loading the page can get visibly slower
      • Adding a defer="defer" argument enables files to be _loaded as resources get freed (=faster!)
      • Still, the files will get executed in the correct order!
    • About file order: load more general stuff first, and the most specialized stuff last.
    • It's also clearer if you make function calls in the last script only , and only __declare __ stuff in the earlier scripts.

    Extra: Executing JS on page load

    To execute script when the page loads, you can do this:

    In your script.js file, you can define a function called, say, load()

    then set it as a value of window.onload

    function load() {

    console.log("page loaded!");

    }

    window.onload = load;

    Exercise 4: Greeting the user

    Separate your scripts into script files and link them to your main page.

    Create a new file user.js

    Into the new file, add a function that changes text on the page to a personified greeting "Welcome, [username]!

    Run the function when the page is loaded.

    Run the function again when the user submits a new username with a form!

    Dynamic CSS

    More about CSS classes

    HTML elements can have multiple class attributes separated with a spaceSubmitCancel

    Applies to both buttons

    .btn {

    border: none,

    }

    Applies to __ __ .btn-red

    .btn-red {

    background-color: red;

    }

    __Applies to __ .btn-blue

    .btn-blue {

    background-color: blue;

    }

    ClassList

    In JS, classes can be added and removed dynamically with classlist (array-like object):

    const button = document.querySelector(".btn-red");

    button.classList.add("btn-blue");

    button.classList.remove("btn-red");

    Or toggled :const anotherButton = document.querySelector("#another-btn");button.classList.toggle("btn-blue");

    Changing the element style

    We can change the style of an element by using its style property.

    First we get the element:

    const someEl = document.getElementById('someEl');

    Then we can change the element's style:

    someEl.style['font-weight'] = 'bold';

    or

    someEl.style.fontWeight = 'bold';

    CSS visibility vs display

    We can change an element's __visibility __ by using the css "visibility" property. Basically meaning that we can either show or hide an element . Let's first store a reference to the element in someEl variable

    const someEl = document.getElementById('someEl');

    Notice that display removes the element and its effects from the DOM

    This frees up space , but the element is still visible in the source code

    __Visibility __ only hides the element from the DOM, still reserving the space for it

    someEl.style.visibility="hidden"; someEl.style.display="none";

    someEl.style.visibility="visible"; someEl.style.display="block";

    Exercise 5: change styles on hover

    Get a handle to the three different sections on your page and make them change color to yellow when you hover your mouse over them

    

    ---

    Solutions: Solutions: https://buuttilab.com/bctjs/week3-html-css-js-dom/-/tree/master/lecture-exercise-solutions/hover

    Dynamic HTML

    Inserting HTML using JS

    JavaScript allows us to create HTML by saving HTML as a string

    let someHtml = "

    Hello there

    ";

    document.getElementById("someElementId").innerHTML = someHtml;

    More often than not, we want to use template literals instead of quotations, since this will help us embed variables in to our HTML code

    let someHtml = `

    Hello there ${friend}

    `;

    Creating HTML elements by using JS

    JS allows us to __create new HTML elements __ programmatically as well

    This way of adding new HTML elements is less prone to typos than when using string literals

    const pTag = document.createElement("p");

    pTag.innerHTML = "I am some Html in a p tag";

    document.getElementById("someDivForExample").appendChild(pTag);

    OR

    const text = document.createTextNode("I am text node to append");

    pTag.appendChild(text);

    document.getElementById("someDivForExample").appendChild(pTag);

    Removing a HTML element by using JS

    Similarly, we can also __remove __ an element from DOM

    const post = document.getElementById("someElementWeWantToRemove");

    const posts = post.parentElement;

    posts.removeChild(post);

    Alternatively, we can remove the n th child from a parent like this:

    const posts = document.getElementById("list_of_posts");

    posts.removeChild(posts.childNodes[n]);

    DOM

    

    Wikimedia Commons

    HTML DOM (Document Object Model) is the tree structure of the web page

    it's created when the web page is opened

    JS can modify all elements within the DOM

    DOM elements are objects , which have a range of different methods and properties

    document.getElementById("idname").innerHTML;

    getElementById _() _ _ is a method , innerHTML _ _ is a property

    Hint: We can access element attributes with method getAttribute()

    Exercise 6: event listeners

    Create a simple user interface that displays jokes. Create four buttons and event listeners for:

    Displaying one random joke

    Displaying one random nerdy joke (category: nerdy)

    Displaying all jokes

    Deleting the first joke

    Jokes array can be found on Gitea .

    ---

    Default joke array and solution: https://buuttilab.com/bctjs/week3-html-css-js-dom/-/blob/master/lecture-exercise-solutions/jokes/main.js

    Chrome Dev Tools

    • The most important tools of a modern web developer are bundled with the browser!
    • Open the _ Inspect _ menu with
    • F12 or CTRL+SHIFT+I
    • right-click the webpage and click Inspect
    • Inspect has multiple tabs
    • Most used ones: _ _
      • Console , Network , Application , Elements, Toggle.

    

    Console

    _Browser has its own _ Console

    one of the most important tools for a front-end developer

    e.g., app's error messages, console.logs and failed requests are printed here

    

    • In Elements you can browse and edit the HTML structure of the webpage
    • easily demonstrate different styles in the browser
      • move the edits into code editor
    • The internal structure of ready-made UI components get exposed here

    

    • Network exposes the data traffic (HTTP!)
      • the requests the page is sending to the server, and their responses
    • Helps e.g., in the optimization of page load time
      • You can see which elements load slower!
    • you can check Disable cache to make debugging easier (no previous data influences the current state of the web app!)
    • check Preserve log to keep the console log intact when the page is reloaded

    

    In _ Application _ you can track and manage e.g.,

    Local Storage

    cookies

    tokens

    

    Toggle

    • Toggle is used for designing responsiveness of webpages
    • How would the page look on…
      • different resolutions
      • phones, tablets, etc…

    

    

    Exercise 7: Let's mess around

    Go to some random news page and change a headline into something silly with the Dev tools. (It's surprisingly easy!)

    

    Assignments

    Assignments about this topic can be found here