The Canvas Element

The <canvas> element represents a raster graphic, much like the <img> element. But instead of representing an existing image file, the <canvas> is a blank slate - a grid of pixels on which you can draw using JavaScript. Becuase a canvas doesn’t determine its size from an image file, you need to always declare it with a width and height attribute (otherwise, it has a width and height of 0):

<div style="border 1px solid gray">
  <canvas id="example-canvas-1" width=400 height=200></canvas>
  <button onclick="fillCanvas">Fill Canvas<button>
  <button onclick="clearCanvas">Clear Canvas</button>

You probably noticed that there appears to be nothing on the page above - but our canvas is there, it is just empty! Clicking the Fill button will fill it with a solid color, and the Clear button will erase it. So how does this work?

To draw into a canvas, we also need a context, a JavaScript object that allows us to draw onto the canvas. We’ll specifically be using the CanvasRenderingContext2D. To get one, we:

  1. Get a reference to the canvas object
  2. Call its getContext() method with an argument of '2d'

The JavaScript to do so looks like:

var canvas = document.getElementById('example-canvas-1');
var ctx = canvas.getContext('2d');

Once we have the context, we can draw onto the canvas using one of its many commands

function fillCanvas() {
  ctx.fillRect(0, 0, 200, 400);

The result can be seen in the canvas above. Next, let’s discuss how the canvas element and context work together.