This MR attempts to implement the idea from#686

It adds astrictMode (false by default, enabled bysetStrictMode or passingstrictMode inImmer constructor) that makes it so that accessing non-draftable properties will throw an error, unless the access is made in anunsafe callback.

Fixes#686

Example

classFoo{}setStrictMode(true)produce({instance:newFoo()},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 inbase.js tests.

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