`"instanceof": "Function"` inside "anyOf" getting converted to `[k: string]: any`

[TheLarkInn]

Before:

   {
    "target": {
      "description": "Environment to build for",
      "anyOf": [
        {
          "enum": [
            "web",
            "webworker",
            "node",
            "async-node",
            "node-webkit",
            "electron-main",
            "electron-renderer"
          ]
        },
        {
          "instanceof": "Function"
        }
      ]
    },
  }

converts (i believe improperly to):

	/**
	 * Environment to build for
	 */
	target?:
		| (
				| "web"
				| "webworker"
				| "node"
				| "async-node"
				| "node-webkit"
				| "electron-main"
				| "electron-renderer")
		| {
				[k: string]: any;
		  };

I identified this throughout this PR for webpack/webpack#7232

[bcherny]

Hi @TheLarkInn! instanceof is not a valid JSON-Schema validator. It looks like it's an AJV-specific extension.

To add support for this, we'd need to implement support for custom JSON-Schema extensions. #2

That said, I think the fix in your case is to not use instanceof if you can. The goal of JSON-Schema is a transport description format that interoperates with lots of environments, both JS and non-JS. For that reason, describing types like functions and classes is really tricky to do right.

[TheLarkInn]

Wow TIL! Thank you for this information! Totally understandable given the constraints.

[TheLarkInn]

I would be awesome to support it but maybe an integration into this system (plugin also) could be a good step. I'll look into this a bit.