JavaScript Lab

The purpose of this lab to practice using JavaScript to create an interactive webpage and introduce new features of the language.


In this lab, we will be creating a website for playing the game “Falling Boulders.” The goal of this game is to move your player across the bottom of the screen using the left and right arrow keys to avoid constantly falling boulders. Every boulder that is successfully dodged earns the player a point. The game ends when the player collides with one of the boulders. You can play a finished demo of the game here.

Step 1: Initial Code

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To start off, you are going to need to download the starter code, which you can download here. This includes an html file (index.html) with the layout of the page already set up and a JavaScript file (framework.js) with some pre-written code that takes care of some behind-the-scenes work we don’t want to have to worry about while coding the game itself.

Once you have that downloaded, go ahead and create a new file called main.js and add a reference to it with the <script> tag at the bottom of index.html. If you don’t know how to do this, look at the tag loading framework.js for reference.

All of the code we write from now on will be inside main.js

Step 2: Initializing Global Variables

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Before we begin coding the game itself, we are going to want to initialize some variables that we will use throughout the project. At the top of main.js, create three variables scoreBoard, startButton, and canvas for storing the scoreboard, start button, and canvas respectively and use document.getElementById() to create references to each of their respective HTML elements in index.html.

Next, create a variable for storing the context of the canvas. This will look something like this:

var ctx = canvas.getContext('2d');

What this line does is give us a way to interact with our canvas. We pass the argument '2d' to let JavaScript know that we want to draw 2-D images onto the canvas.

Now that we have these variables, let’s use them to code our game.

Step 3: Begin Creating the Player

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The first part we want to create is our player object. On the next line, create a new object representing our player and store it in a variable called player. For now, we will give our player object three properties: hitbox, color, and draw.

First, let’s assign a new Rectangle object to hitbox. The constructor for Rectangle is already defined in framework.js and is used as follows:

var rect = new Rectangle(x, y, width, height);

This rectangle is going to represent the bounds of our player for detecting collisions and for drawing the player on the canvas.

Assign values for x and y so that the player will appear on the bottom of the canvas and give it a width and height of your choosing (in my example, I used a width of 30 and a height of 60). Keep in mind that for canvases the top left corner is the origin (0, 0). Also, you may find it useful to use canvas.width and canvas.height in your calculations.

A Note about the Canvas

Throughout the lab, it will be helpful to think about the canvas as being a grid of 10x10 cells. For example, this is why I gave my example player a width of 30 and height of 60. This corresponds nicely to a 3x6 block of cells. Keep this in mind when creating other elements later.

Next, let’s give our player a color. Assign a String to the color property of your player object to represent whatever color you want. This can be a color name, such as "red" (if it’s supported by HTML), or a hex code, such as #FF0000.

Finally, for draw we are going to write a function that will draw the player onto the canvas. For now, this will be pretty basic and just draw the hitbox in the color you chose previously. To do this, call the draw() method on the hitbox, which looks like:

rect.draw(ctx, color);

where ctx is the context of the canvas we wish to draw on (which we created in Step 2) and color is a string representing the color of the rectangle. Fill in these values as well as the value of rect yourself. (Hint: use the this keyword)

Step 4: Rendering the Player

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Now that we have a player with a draw function, let’s display the player on the canvas. To do this, we are going to assign a new function to document.body.onload like this:

document.body.onload = function()

So, what does this do? Well, document.body refers to the <body> of our HTML page and onload is property that stores a function to be called as soon as the webpage loads. document.body.onload is null by default, so by setting it to our newly created function we are telling the page that we want this new block of code to be run once the page finishes loading. This will sort of act similarly to a main method in Java or C.

Now, we can begin seeing the fruits of labor! Open up a web browser and click File > Open File (ctrl/cmd + O), navigate to wherever you saved this project, and select index.html. If your code is all correct, you should see a colored rectangle on a blank gameboard. (If you don’t see this, check the JavaScript console for errors).

From here on out, we will be able to test our code simply by going to the browser and refreshing the page.

Step 5: Drawing a Prettier Player (Optional)

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At this point we have a pretty basic player displaying on the gameboard. But what if our player aspires to be more than just a rectangle? Well, we can fix that!

Back in the player object, add a new property called body. body is going to be an array of rectangle objects that represent different pieces of our player’s body. This can look like however you want, just keep in mind it should fit the bounds of hitbox. Also, consider the top left corner of hitbox to be the coordinate (0,0) when assigning x and y values to your rectangles. The reason for this will be apparent in the next part of this step.

Since we’ve changed what our player looks like, we’re going to want to rewrite the draw function. Instead of calling Rectangle.draw() on just hitbox, loop through the rectangles in body and call Rectangle.draw() on each one of them. However, this time we are going to add an argument as follows:

rect.draw(ctx, color, offset)

offset is an optional parameter that offsets the rectangle by the x and y values of the rectangle passed in (this is why we treated the top left corner of hitbox as (0,0) when making body). Fill in rect, ctx, and color as you did previously, but for offset fill in the value this.hitbox.

Step 6: Moving the Player

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So now we have a player displaying, but what good is a player if it can’t move? Our next step will be to add moveLeft() and moveRight() functions to move the player across the bottom of the gameboard.

Before we write these functions, we are going to first define a function erase() inside player. The reason for this is that we want to be able to update the player’s location on the canvas after we move it and the only way we can do this is by erasing the player from its previous location then redrawing it in the new one. Similarly to draw(), we already have a function defined in framework.js for erasing that can called like:

clearRect(ctx, rect);

Again, call clearRect() in your erase() function replacing ctx and rect with the correct values and you are done.

Now, let’s write moveLeft() and moveRight() (still inside player). I’ll leave the implementation to you, but each function should fulfill the following tasks:

  1. Erase the player from the canvas
  2. Move the player’s location (hint: edit some property in hitbox)
  3. Make sure the player cannot leave the bounds of the screen
  4. Redraw the player onto the canvas

When choosing the amount to shift the player by, keep in mind the hypothetical grid of 10x10 cells discussed in Step 3.

Step 7: Listening for Key Presses

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Remember when we assigned a function to document.body.onload to run some code when the page loaded? Well, we can edit a similar property to have code whenever a key is pressed on the keyboard. Inside the function we assigned to document.body.onload, add the following snippet:

document.body.onkeydown = function(e)
    var keyCode = e.which || e.keyCode;
    // to be implemented

Notice that for this function we have defined a parameter e. This refers to the event data that will be passed by the browser when it calls the function, which is how we determine which key was pressed.

This determination is performed in the line var keyCode = e.which || e.keyCode;. Similarly to how characters have an ASCII value, every key on the keyboard has its own key code that can be used to identify it. This value is stored in either e.which or e.keyCode depending on which browser you are using. That’s why we include e.which || e.keyCode. What this does is check if e.which is defined: if it is, then its value is returned, if not, then e.keyCode is returned.

Now that we have the key code, we can check if the left or right arrow keys have been pressed. If the left arrow key is pressed, the player should move left; otherwise, if the right arrow key is pressed, the player should move right. Implement this yourself. (Note: left arrow has a key code of 37, right arrow is 39)

Step 8: Constructing Boulders

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At this point, you should have all the skills necessary to write a constructor for Boulder objects yourself. Like player, it should have x, y, color, hitbox, draw, and erase properties. color should be passed as a parameter into the constructor so that we can easily change it (or even have differently colored boulders!). The x coordinate should be given a random position on the canvas such that it is fully visible (hint: use Math.random()) and the y coordinate should be 0. Since our boulders are just squares, just having hitbox will suffice to represent both the bounds and the shape we want displayed. draw and erase should look pretty similar to those of player.

Keeping with the idea of 10x10 cells, I recommend making the boulders 10x10 and making their x values multiples of 10.

Step 9: Falling Boulders Part 1

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Now that we have our Boulder constructor, let’s start making boulders. First, we need some variables to store our boulders. Under the global variables we created at the top of the file, add two more variables: boulderCount, which is how many boulders we want in existence at one time, and boulders, which should be initialized as a empty array. Give boulderCount any value you want; I chose 15 in my example.

The rest of this step is going to be creating a new function called start. The purpose of start is to create new boulders after a time interval until we reach the size of boulderCount.

Inside start, initialize a new variable to 0 to represent a counter. Next, we are going to use JavaScript’s setInterval() function to create new boulders every second or so. Add the following code underneath your counter:

var boulderInterval = setInterval(function() {
  // to be implemented
}, 900);

This will run the code inside the anonymously defined function every 900 milliseconds forever or until we terminate the interval. To terminate it, call the function clearInterval(boulderInterval). Inside the new function for our interval, check if the counter is equal to boulderCount and terminate the interval if it is. Otherwise, add a new boulder to boulders with a color of your choosing and increment the counter. Note that right now we are only creating boulders, not drawing them. Drawing will come in the next step.

Finally, we want start to run when the user presses the start button on the webpage. Back inside the function we assigned to document.body.onload, add the following line:

startButton.onclick = start;

This tells the website that whenever someone clicks the start button, we want our start function to be called. However, we really only want start to be called the first time the start button gets clicked. So, at the top of start, add a similar line:

startButton.onclick = null;

This will stop start from being called again when the start button gets clicked.

Step 10: Falling Boulders Part 2

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In Step 9 we created several boulders, but we haven’t made them fall yet. Let’s write a new function called fall to do just that. Here’s what fall should do: for each boulder in the array boulders,

  1. Erase the boulder from the canvas
  2. Update its y value (again, I recommend adding 10)
  3. If a boulder reaches the bottom, replace it with a new a boulder starting at the top
  4. Redraw the boulder

Once you finish that, we need to call fall. At the top of the file, add another global variable called fallInterval and set it to null. Next, add the following line to the end of start:

fallInterval = setInterval(fall, 40);

This makes it so fall gets called every 40 milliseconds. We store the interval in fallInterval so that we can clear it later.

Step 11: Keeping Score

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Our game is almost complete! All we have left is to keep track of score and determine when the game is over.

Create another global variable called score and initialize it to 0. Next, write a new function called updateScore. This function is pretty simple: all it does is increment score by 1 and display the new score on the screen. To display the new score, edit the innerHTML property of scoreBoard as follows:

scoreBoard.innerHTML = "Score: " + score;

Next, write a new function called checkHit. This will be how we determine when to the player has lost. In the function, for each boulder in boulders, check if its hitbox intersects the player’s hitbox with the intersects() function from our rectangle framework which looks like:


and returns true if the rect and rect2 are intersecting. If the player has been hit, clear fallInterval and let the player know that the game is over using JavaScript’s built-in alert() function. Pass a String with a message telling the player that they lost and letting them know their final score into alert (e.g. alert("Game Over")) and the page will display a popup with your message. Finally, we want to refresh the page so that the player can play again. To do this, all we need is the line


Now that we have updateScore and checkHit, let’s put them inside fall. At the top of fall call checkHit and inside your check for if a boulder reached the bottom call updateScore if it in fact hit the bottom.

Congratulations, you’ve finished your first game in JavaScript!

Written by Ari Cohn, October 2017