WebGLRenderer: Fix for runtime error that happens when null is provided as context

[soadzoor]

Description

If someone passes a context to the WebGLRenderer, but the context is null (because the user's device doesn't support webgl, or webgl2, or whatever context they tried to create), we have a runtime error, saying TypeError: _context is null.

The problem was introduced with this commit: fb9d575

To make things a little bit more confusing, the official documentation says that the default value for context is null: https://threejs.org/docs/index.html?q=webg#api/en/renderers/WebGLRenderer

Which is, of course, technically incorrect. It's not null, but undefined by default. The root of the problems is that some of the checks inside the WebGLRenderer's "constructor" is checking whether the context is undefined. If it's not, it assumes it has a valid value, although it can be easily null. Because canvas.getContext either returns a context, or null.

For a long time this was the official recommended way for creating WebGLRenderer with WebGL 2.0 context:

var context = canvas.getContext( 'webgl2' );
var renderer = new THREE.WebGLRenderer( { canvas: canvas, context: context } );

I know this can be now considered "legacy code", but still, if webgl2 is not available on a device, and someone still uses the code like this, it will give the user an error instead of trying to fall back to webgl 1.

This PR replaces the strict comparisons context !== undefined with context != undefined, and provides a technically more accurate default value in the documentation.

[Mugen87]

To make things a little bit more confusing, the official documentation says that the default value for context is null

I think the docs are right. The line _context = parameters.context !== undefined ? parameters.context : null, is in some sense a replacement for the default parameter syntax.

The better change is replacing the following line:

three.js/src/renderers/WebGLRenderer.js

Line 72 in 01e4eaf

if ( parameters.context !== undefined ) {

with

if ( _context !== null ) {

BTW: We do not use != in general. Better to make the check more explicit.

[soadzoor]

@Mugen87 Alright, I'm fine with that, I modified the PR according to it

[WestLangley]

BTW: We do not use != in general.

Agreed. It would be a good idea to grep for those and fix them. 😇

Also, ==.

[mrdoob]

Thanks!