6. Interacting with the DOM

Lesson objectives

  • Students learn how to use JS to update the html page (DOM, update input field/element colour)

  • Students learn how to use JS to handle html events (button click)

  • Students learn how to use JS to navigate to a different page

  • Students learn how include JS file in a website

Lesson time to deliever:

1 sessions, 1.5hrs

Lesson content


If an element has the id attribute, we can get the element using the method document.getElementById(id), no matter where it is.

<div id="elem">
  <div id="elem-content">Element</div>

  // get the element
  let elem = document.getElementById('elem');

  // make its background red
  elem.style.background = 'red';

The id must be unique

The id must be unique. There can be only one element in the document with the given id.

If there are multiple elements with the same id, then the behavior of methods that use it is unpredictable, e.g. document.getElementById may return any of such elements at random. So please stick to the rule and keep id unique.

(maybe don't cover as can be confusing): Also, there’s a global variable named by id that references the element. So can just do:

<div id="elem">
  <div id="elem-content">Element</div>

  // elem is a reference to DOM-element with id="elem"
  elem.style.background = 'red';

  // id="elem-content" has a hyphen inside, so it can't be a variable name
  // ...but we can access it using square brackets: window['elem-content']


By far, the most versatile method, elem.querySelectorAll(css) returns all elements inside elem matching the given CSS selector.

Here we look for all <li> elements that are last children:

  let elements = document.querySelectorAll('ul > li:last-child');

  for (let elem of elements) {
    alert(elem.innerHTML); // "test", "passed"

Setting value of input field

<input type="text" id="mytext">
document.getElementById("mytext").value = "My value";


An event is a signal that something has happened. All DOM nodes generate such signals (but events are not limited to DOM).

Here’s a list of the most useful DOM events:

  • click – when the mouse clicks on an element (touchscreen devices generate it on a tap).

  • keydown and keyup – when a keyboard key is pressed and released.

  • DOMContentLoaded – when the HTML is loaded and processed, DOM is fully built.

Handle button click:

<input value="Click me" onclick="alert('Click!')" type="button">

Do a demo using button click to change the webpage:

 <button id='elem'>Click me</button> 
 let elem = document.getElementById('elem');
 elem.addEventListener("click", changePage); // can do like this
 elem.onclick = function(){changePage}; // or this way
 function changePage(){
   window.location.href = "https://stackoverflow.com/";


  • Get to update login page to add js file

  • In js file update login button click to validate input to ensure email contains ...

  • Clarify this is not how logins work as can see correct answer in code

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