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window.onload = canvasApp();
We will use the first method throughout this topic.
Encapsulating Your JavaScript Code for Canvas
Now that we have created a way to test to see whether the HTML page has loaded, we
can start creating our JavaScript application. Because JavaScript runs in an HTML page,
it could be running with other JavaScript applications and code simultaneously. Usu-
ally, this does not cause any problems. However, there is a chance that your code might
have variables or functions that conflict with other JavaScript code on the HTML page.
Canvas applications are a bit different from other apps that run in the web browser.
Because Canvas executes its display in a defined region of the screen, its functionality
is most likely self-contained, so it should not interfere with the rest of the page, and
vice versa. You might also want to put multiple Canvas apps on the same page, so there
must be some kind of separation of JavaScript when defining the code.
To avoid this issue, you can encapsulate your variables and functions by placing them
inside another function. Functions in JavaScript are objects themselves, and objects in
JavaScript can have both properties and methods. By placing a function inside another
function, you are making the second function local in scope to the first function.
In our example, we are going to have the canvasApp() function that is called from the
window load event contain our entire Canvas application. This “Hello World!” example
will have one function named drawScreen() . As soon as canvasApp() is called, we will
call drawScreen() immediately to draw our “Hello World!” text.
The drawScreen() function is now local to canvasApp() . Any variables or functions we
create in canvasApp() will be local to drawScreen() , but not to the rest of the HTML
page or other JavaScript applications that might be running.
Here is the sample code for how we will encapsulate functions and code for our Canvas
function canvasApp() {
function drawScreen() {
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