Shape Functions

For ease of use, the context also supplies a number of functions for drawing shapes. Some of these just define the shape as a series of subpaths to be used with the stroke() and fill() functions.

Arcs and Circles

Of these, we’ve already seen the arc(x, y, radius, startAngle, endAngle) function. It can be used to draw an arc, or when filled, a wedge - like a pie slice. When a startAngle of 0 and endAngle of 2 * Math.PI is used, it instead draws a complete circle.

<canvas id="arc-example" width="500" height="200"></canvas>
  var canvas = document.getElementById('arc-example');
  var ctx = canvas.getContext('2d');
  ctx.strokeStyle = 'green';
  ctx.arc(100, 100, 50, 0, Math.PI * 2);
  ctx.fillStyle = 'purple';
  ctx.arc(250, 100, 50, 0, Math.PI * 2);

  ctx.moveTo(400, 100);
  ctx.arc(400, 100, 50, Math.PI, Math.PI / 4);


There is also an ellipse(x, y, radiusX, radiusY, startAngle, endAngle) function. It works in a similar way, creating an ellipse centered on the point (x, y) with a horizontal radius of radiusX and a vertical radius of radiusY, and starting and ending angles of startAngle and endAngle (specified in radians). As with the arc() method, a startAngle of 0 and endAngle of 2 * Math.PI creates a full ellipse.


The rect(x, y, width, height) function draws a rectangle with upper left corner at (x, y) and dimensions width and height. In addition to the rect() function, there are some shorthand functions that also stroke or fill the rect without altering the current path.

The first of these is fillRect(x, y, width, height) which uses the same arguments as rect() and fills the resulting rectangle with the current fill style. We used this in the introduction.

The second is the clearRect(x, y, width, height) function, which fills the rect defined by the arguments with transparent black (rgba(0,0,0,0)), effectively erasing everything in that rectangle on the canvas.


Why the extra rectangle shorthands, and not other shapes? Remember, computer graphics have traditionally been rectangular, i.e. windows are rectangles. So programmers have gotten used to using rectangles in a lot of places, so having the extra shorthands made sense to the developers. Also, both fillRect() and clearRect() are used to quickly clear <canvas> elements of any prior drawing, especially when creating canvas animations.