Note: This feature is available in Web Workers.
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.
A number.
Given this <canvas> element:
<canvas id="canvas"></canvas>You can get the width of the drawing buffer with the following lines:
const canvas = document.getElementById("canvas");
const gl = canvas.getContext("webgl");
gl.drawingBufferWidth; // 300