You can think of functions as little robots that go off and do things for you.
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One function might fetch you something, another one might calculate something, and another one might display something for you. Why are objects also called dictionaries? Because dictionaries work like this too. Give them a word and they give you back a corresponding definition. Every search works like this too.
If you type in a word or phrase into Google, you get back a bunch of results, right? The Boston Public Library works like this, except it uses the Library of Congress Classification system, which uses call numbers as keys.
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People work like this too. Think of all the things people could look you up by. Your address, your phone number, your social security number. These are all keys, which, when passed into an object, might return a person like you. The basic structure looks like this:. So, you plug the key — i. On the left of the colon is the key, on the right in the value.
This list is just a list — otherwise known as an array. Lists of friends on Facebook, lists of followers on Twitter or Instagram, lists of notifications in Snapchat, lists of movies on IMDB, lists of email messages in Gmail. Strings are words or phrases that you want to use in your program, most likely to display to the people using your program. So, if you want to say hello or make a warning pop up, you have to write your words in quotes, like this:.
Next is numbers. Comparisons are also used in our daily life. This is a hard one because it might not make sense why we need them at first, but let me explain. You could store that away and then, later on, greet them by their name. Variables are also really useful for keeping track of things you might be confused about when come back to your code later. You can use variables to name values, so your program makes more sense when you come back to it.
However, if you label each number it might make more sense:. Labels help you keep track of that. If there were no bad guys and the whole game was just a flat path that you could walk on endlessly, it would be really boring… This is the other thing variables are good for — because you can change their values depending on the situation.
That would make it really hard for the boat to sail against the wind. They might need to boost their speed somehow! They let us put in a key and get back a value? Well, there are a few of those here. So, document here is an object. We can access the value of the body key inside of this object by typing:.
What does the value of body equal? Remember when we talked about functions? In this case, the addEventListener function helps us figure when an event on the page occurs. This could be a mouse click or scroll event, someone pressing a key on a keyboard, or someone writing text in a text area. Now, how can a function tell us when something has happened or give us data we requested from us? One way would be to pop something up on the screen for us. This could be done by the function calling alert inside of itself. Another way a function could return data to us — in fact the most common way — would be to just give us a result back from the function, which we can assign to a variable.
This is called returning a value, and when a function does this, we can get the value it returns like this:. This is great for most types of functions. You can forget about them until the next time you need them. However, sometimes you need new data every time an event happens. In this case, a very common pattern is to pass a function into another function. Then, the function you pass in will be called whenever the event happens, on the dot. The second argument to addEventListener is a function that will get called whenever someone presses a key on the keyboard.
If we want the function to have a name, so we can refer to it or call it later, we can write it like this:. Okay, so, to review: we have an object, document , that has another object inside of it, body , that has a function inside of it, called addEventListener. The second argument is a function, that we define, that will be called whenever that event takes place. So, in this case, when a user presses a key on the keyboard, the function we passed to addEventListener will be called. And, to top is off, when addEventListener calls our little function, it will also be nice enough to give us some of the details about the event.
In the example, we refer to these details with the name event. We check to see if the value of keyCode represents the key for the right arrow i.
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In plain English: - We ask the computer to tell us when someone presses a key. The reason for using a library is this: when something has been built thousands of times before like games and game libraries have , you can learn a lot from established solutions and get a lot of the hard work done for you for free. If you want to build a game from scratch, instead of using a library, it will be a lot easier for you to do that after understanding the basic principles of game development.
The first thing to notice is that we already have a working animation! And, with very little code! Each of these concepts is super simple on its own, but when you put them together you have the basics for making a game. Sprites are items or characters or tiles on a map that you want to display on a screen. They might be a shape or they might be a drawing or icon. This object sets up all the initial settings for our sprite. Coordinates are just where the sprites appear on the screen. If we have a sprite that moves to the right, we need to know where it starts, so we can tell it where to go next.
With this game library, the 0,0 coordinate is in the top left. That means the further the sprite gets from the top left corner, the higher these coordinates go. The first coordinate is the x coordinate and it represents how far the sprite is from the left side of the game board. The second coordinate is the y coordinate and it represents how far the sprite is from the top of the game board.
Creating the canvas
Show all. Table of contents 14 chapters Table of contents 14 chapters Introduction Pages Pitt, Christopher. Player Input Pages Pitt, Christopher. Collision Detection Pages Pitt, Christopher. Gravity Pages Pitt, Christopher. Ladders Pages Pitt, Christopher. Stairs Pages Pitt, Christopher. Camera Locking Pages Pitt, Christopher. Projectiles Pages Pitt, Christopher.