This MR attempts to implement the idea from #686

It adds a strictMode (false by default, enabled by setStrictMode or passing strictMode in Immer constructor) that makes it so that accessing non-draftable properties will throw an error, unless the access is made in an unsafe callback.

Fixes #686

Example

class Foo {}

setStrictMode(true)

produce({ instance: new Foo() }, draft => {
  unsafe(() => {
    draft.instance.abc = 1; // works fine in unsafe
  });

  console.log(draft.instance); // will throw, because access not wrapped in unsafe
});

See the tests for more examples

Limitations

I tried to make it as compliant with existing functionalities as possible. I was not able to make it fully ES5-compliant. There are a few tests that fail when strict mode is enabled in base.js tests.

I do not have enough expertise to fully fix them. Suggestions are welcome