Canvas reference: The getContext function

Summary
The getContext function returns the drawing context - which is an object that has all the drawing properties and functions you use to draw on the canvas.

The getContext function is the function that you use to get access to the canvas tags 2D drawing functions. As of April 2014 there are two types of context that are available to you: 2d and webgl. These provide you with the API that you use to draw on the canvas. It should be noted that the webgl context is significantly more complicated than the 2d version.

A more recent addition is the ability to give a second argument of options (an object) - of which currently the only available option is the alpha setting. By setting this you're saying that the canvas has no transparency and the background is initialised to black. You can still use semi-transparent colors on top of that - but keep in mind that the background is now black.

Arguments to the function

• The type of drawing context that you want. This can be 2d or webgl (or experimental-webgl for older browsers).
• An optional object of options to give the context. Currently (April 2014) there is only one option - alpha - which stipulates that the background of the canvas is not transparent. In this case it is initialised to the color black. See the example below for how to give this option.

An example

<script>
    window.onload = function ()
    {
        var canvas  = document.getElementById("cvs");
        var context = canvas.getContext('2d');

        // Fetch the context and stipulate it as having no alpha/opacity
        // var context = canvas.getContext('2d', {alpha: false} );
    
        // var context = canvas.getContext('webgl');              // WebGL 3D ontext
        // var context = canvas.getContext('experimental-webgl'); // Older WebGL 3D context
    }
</script>