WebGLRenderingContext.drawingBufferWidth

The read-only WebGLRenderingContext.drawingBufferWidth property represents the actual width of the current drawing buffer. It should match the width attribute of the <canvas> element associated with this context, but might differ if the implementation is not able to provide the requested width.

Given this <canvas> element:

<canvas id="canvas"></canvas>

You can get the width of the drawing buffer with the following lines:

var canvas = document.getElementById('canvas');
var gl = canvas.getContext('webgl');
gl.drawingBufferWidth; // 300

• WebGLRenderingContext.drawingBufferHeight
• WebGLRenderingContext.viewport()