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Theme: Next, 1

[fit] TypeScript and the DOM

[fit] Tic Tac Toe

Let's create an interactive Tic Tac Toe game for two players.

As always, we will start with static HTML and CSS

<!DOCTYPE html>
<head lang="en">
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Tic Tac Toe</title>
<link rel="shortcut icon" href="/favicon.ico" />
<h1>Tic Tac Toe</h1>
<script type="module" src="/src/main.ts" charset="utf-8"></script>

CSS Root Variables

:root {
/* CSS Variables for all the font colors and sizes. Try changing these! */
--header-background: #5661b3;
--header-text-color: #fff9c2;
--header-font-size: 2rem;
--square-font-size: calc(8 * var(--header-font-size));
--square-text-color: #5661b3;
--square-background-color: #e6e8ff;
--square-border: 3px solid var(--square-text-color);
font: 16px / 1 sans-serif;

CSS html/body formatting

html {
height: 100%;
body {
margin: 0;
min-height: 100%;

Style the header

h1 {
/* center the header */
text-align: center;
/* Use a sans serif font with a little spacing and color */
font-family: Verdana, Geneva, Tahoma, sans-serif;
letter-spacing: 0.4rem;
font-size: var(--header-font-size);
color: var(--header-text-color);
/* Remove margins and set a little padding */
margin: 0;
padding: var(--header-font-size);
/* Set a background color for the header */
background-color: var(--header-background);

Reset ul/li styles

li {
/* Be gone margins! */
margin: 0;
padding: 0;
/* and list styles */
list-style: none;

Grid format the main ul

ul {
/* Make the height of the list equal to the height of the page MINUS the height taken by the header */
height: calc(100vh - 3 * var(--header-font-size));
/* Display the list as a 3 column and three row grid */
display: grid;
grid-template: 1fr 1fr 1fr / 1fr 1fr 1fr;
/* Add a little gap between to allow the background color through */
gap: 1rem;
/* Set the background color that will show through the gap */
background-color: var(--square-text-color);

[fit] Style the li, in our case, the Tic Tac Toe cell

ul li {
/* Use a monospace font */
font-family: monospace;
font-size: var(--square-font-size);
/* Style the background color of the item */
background-color: var(--square-background-color);
/* Make the cursor a pointer by default */
cursor: pointer;
/* Center the text in the LI */
display: flex;
align-items: center;
justify-content: center;
/* Don't let the squares become too small */
min-width: 3rem;
min-height: 10rem;

[fit] Add styles for cells that are taken and not-allowed

ul li.taken {
cursor: not-allowed;
ul li.small {
font-size: 4rem;
ul li.not-allowed-click {
background-color: red;

Let's add an "X" to one of our cells and mark it as taken

[fit] Could we dynamically change the content?

Time to make our page reactive by adding in TypeScript that interacts with the browser's content

TypeScript, when run in the browser, has access to the DOM (Document Object Model)

Remember that Elements view in the web developer's tool?

That isn't our index.html file! The browser's interpretation of the content into a nested set of objects the browser uses to render our page.

But wait! We know how to use objects in TypeScript!

If we had a way to access those live objects in our code, we could adjust them.

If we can adjust them, the browser would show us the updated view!

Enter the DOM and the TypeScript DOM API!


Open up developer tools

We've seen some "built-in" objects already.






window.alert('Watch out!')
const name = window.prompt('Your name?')

What about interacting with the current "document"?

In the console, type: document and press enter


This document is the LIVE view of our HTML code

Our browser loads the HTML and creates objects for each element

The DOM (Document Object Model) is the browser's view of the live content

How do we get access to these elements so we can change them?

document gives us methods to start finding (`select'ing) elements.

So many options!

There are many functions the document can do, but querySelector is the most basic

[fit] querySelector Looks for the FIRST matching element

document.querySelector( ... css selector syntax as a string ...)



Will return the first li element in the document, scanning from top to bottom.

Gives back a real object, something we could assign to a variable!

This opens up an entire world!

We didn't get a string or a number; we received an HTMLElement

Dev tools show it formatted like HTML

  • We can expand it to show all the element's attributes (Wow, so many!)
  • We can hover it to highlight it in the browser window
  • We can click on it to jump to that element in the Elements tab

Put it in a variable

firstListItem = document.querySelector('li')

While there are many attributes to investigate, let us start by looking to see what text is inside this element.


Since the li is empty, we get back an empty string!

Update the HTML code to put an `O' in the first li

Rerun the example, and we'll see this code return "O"

firstListItem = document.querySelector('li')

But that isn't all! We can change the textContent

firstListItem.textContent = 'X'

Whoa! The O became an X

We could even add to the text

firstListItem.textContent = firstListItem.textContent + '*'


firstListItem.textContent += '*'

But what about getting and changing attributes? (e.g. 'class')

Update the original HTML to put a taken class on that first LI.

Now in the console we can do:

firstListItem = document.querySelector('li')

Wow, we get back taken, which is the class name we assigned!

But what if there is more than one class on the item?

Enter classList

firstListItem = document.querySelector('li')

Put two classes in the HTML

<li class="taken small">X</li>

Now see that the classList has two elements!

[.autoscale: true]

Manipulate the class list!

  • We can ask the class list if it contains a class:
  • We can also remove a class!
  • We can also add a class. We can put back the small class
  • Since we add/remove classes so often, we can even toggle them on and off

Now, let us interact!

We can now:

  • Find an element
  • Get its content
  • Change its content
  • Add classes, remove classes, toggle classes

How can we interact with the element? Functions!


function handleClickSquare() {
window.alert('You did it!')

Tell the li: Every time a click happens, here is what to do...

The browser is sitting there waiting for events to happen.

The browser is event driven

[fit] We can teach the firstListItem object new tricks

// What object
// |
// | please listen for an event
// | |
// | | name of event to wait for
// | | |
// | | | function to call when it happens
// | | | |
// v v v v
firstListItem.addEventListener('click', handleClickSquare)

[fit] firstListItem.addEventListener('click', handleClickSquare)

This means:

The list item in the variable firstListItem should listen for a specific kind of event, in this case, a click, and when it happens, the browser will CALL BACK our handleClickSquare function

There is a list of pre-defined events that the elements know!

It is a long list, but you don't need to know them all.

[fit] Try clicking on that element!

[fit] Let us try a different event: Mouse Move

function handleMouseMove() {
firstListItem.addEventListener('mousemove', handleMouseMove)

Improve handleClickSquare to make it DO something

Reload the page to wipe out our work and start clean.

firstListItem = document.querySelector('li')

Now we will redefine handleClickSquare but add a parameter (argument) to our function.

function handleClickSquare(event) {
// Code goes here

Every time our browser handles an event

... we get our function called AND we get an event object!

Let us check out that event!

function handleClickSquare(event) {
firstListItem.addEventListener('click', handleClickSquare)

The event knows which element generated the click!

Check out the target property of the event!

It is the element we clicked on!

Rewrite handleClickSquare to change the text


firstListItem = document.querySelector('li')
function handleClickSquare(event) {
const thingClickedOn =
thingClickedOn.textContent = 'X'
firstListItem.addEventListener('click', handleClickSquare)

[fit] NICE!

However, how do we do this for ALL the list items?


const allSquares = document.querySelectorAll('li')

Try to write allSquares.addEventListener

:x: :x: :x: :x: :x:

We can't directly call it

querySelectorAll returns a NodeList, so we have to loop...

forEach will work!

allSquares.forEach(square =>
square.addEventListener('click', handleClickSquare)

The loop adds an event listener to each item!

[fit] Done playing in the console, to the editor and TypeScript!

Now that we have some experience let us put it in our actual TypeScript file!

Bring our code over to main.ts

import './style.css'
function handleClickSquare(event: MouseEvent) {
// Get the target of the click
const thingClickedOn =
thingClickedOn.textContent = 'X'
const allSquares = document.querySelectorAll('li')
allSquares.forEach(square =>
square.addEventListener('click', handleClickSquare)

There are red-squiggles!

[fit] Telling TypeScript to check!


import './style.css'
// Defines a method for us to handle the click
function handleClickSquare(event: MouseEvent) {
// Get the target of the click
const thingClickedOn =
// If the thing clicked on is an LI Element
// - This does several things:
// - 1. Checks that the target isn't null
// - 2. Let's TypeScript know that *inside* this if statement
// the thingClickedOn is an LI element, and thus we can
// change its textContent
if (thingClickedOn instanceof HTMLLIElement) {
thingClickedOn.textContent = 'X'
const allSquares = document.querySelectorAll('li')
allSquares.forEach(square =>
square.addEventListener('click', handleClickSquare)


Hover over thingClickedOn before and after the if!

TypeScript allows for type narrowing.

The more we tell/test the more TypeScript can know!

[fit] Load the page and see our logic works each time we reload the page!

But our game isn't great.

X keeps getting to take squares!

We should toggle between players!

Time for STATE

let currentPlayer = 'X'
function handleClickSquare(event: MouseEvent) {
const thingClickedOn =
if (thingClickedOn instanceof HTMLLIElement) {
thingClickedOn.textContent = currentPlayer

Ok, but we need to change who's playing after each click!

Bring in some behavior

// If currentPlayer is precisely the text 'X', make the currentPlayer 'O'
if (currentPlayer === 'X') {
currentPlayer = 'O'
} else {
// Otherwise it was already 'O', so make it an 'X'
currentPlayer = 'X'

But what if we made a mistake!

// If currentPlayer is precisely the text 'X', make the currentPlayer 'O'
if (currentPlayer === 'X') {
currentPlayer = 'O'
} else {
// Otherwise it was already 'O', so make it an 'X'
currentPlayer = 'Y'

Make the definition of currentPlayer more specific!

let currentPlayer: 'X' | 'O' = 'X'
This means that currentPlayer is a string, but can *only* be the string X or the string O
TypeScript will check to make sure this is always true in our code, but only during coding and compiling!

[fit] Get clicking!

[fit] What is happening?

+--------------------+ +----------------------+
| Wait for a click | <-- | Go back to waiting |
+--------------------+ +----------------------+
| ^
| |
v |
+---------------------+ +------------------------+
| Browser calls us! | | Toggle currentPlayer |
+---------------------+ +------------------------+
| ^
| |
v |
+---------------------+ +----------------------+
| handleClickSquare | --> | Change textContent |
+---------------------+ +----------------------+

[fit] Oooops, we can click a square twice!

First, let us mark the square as taken!

Add the taken class to an element.


We can add logic/behavior to block clicking twice!

Before we do any work in handleClickSquare


  • Look at the li
  • If the li contains the class taken
    • ... then do nothing, stop work!
  • Otherwise, proceed...

[fit] Guard clause

if (thingClickedOn.classList.contains('taken')) {

Except for winning/losing, we are looking pretty good!

Let us add a counter of the number of moves!

MORE state

let moveCounter = 0

Then every time we make a good move:

// Increment the move counter

See in the console we can look at the moveCounter as we click.

Change the header

First, query for the element

// Get the header to query for the first `h1`
const header = document.querySelector('h1')
// Interpolate a string with the header and the count of moves
// and replace the text content of our header!
header.textContent = `Move ${moveCounter} of Tic Tac Toe`

What have we added to our tool belt?

  • document.querySelector
    • Returns the first element that matches the given selector
  • document.querySelectorAll
    • Returns ALL the elements that match the given selector
  • element.addEventListener
    • Waits on a specific event to happen to a specific element and calls a supplied function

That is all!

With these simple tools, we can make any page dynamic.

... However, there are better tools ...

Bonus Content!

Event bubbling!

Remember when we had to add event listeners to all the LI to handle the clicks?

We did not know that the click on an li will move up the DOM tree to the parent, grandparent, and the great grandparent, hoping someone will be listening to that event.

[fit] Instead of querySelectorAll('li') we can get the ul

Delete the code relating to allSquares and replace it:

const gameBoard = document.querySelector('ul')
gameBoard.addEventListener('click', handleClickSquare)

But wait ...

When a li click happens, it will "bubble up" to the ul

Our handleClickSquare is waiting for it!

Since the will be the clicked li, all our code will work!

It is possible to click on the ul itself. Click on the gaps between squares.

Luckily our instance on type safety helps us! We are already testing for clicks against an <li>!

Event bubbling can reduce the amount of code, but it can be tricky.

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