Jest 25 invokes setters during toEqual comparison, causing side effects

[gaearon]

This is similar to #9531.

It is not safe to assign a bunch of setters in the passed object. They may throw or they may have side effects. This isn't theoretical because we may add setters like this to React.

Example: https://repl.it/repls/HuskyCandidConfiguration

test('setter object comparison', () => {
  expect({
    get a() {
      return {};
    },
    set a(value) {
      throw new Error('noo');
    },
    b: {},
  }).toEqual({ 
    a: {},
   });
});

This used to work fine in 24: https://repl.it/repls/AllMediumorchidDeals

This regression is currently blocking upgrading React to Jest 25.

[SimenB]

I'm guessing we should just try-catch attempting to override, and if we can't do that just keep going leaving the property as it is? We're just trying to give a nicer diff, if we can't do that we should bail and leave things alone. One downside of that is that then we won't know we failed so can't fix it, but probably worth it. Unless we think this is the only case we haven't covered, which seems optimistic

/cc @WeiAnAn @jeysal @pedrottimark

[gaearon]

It’s not just about catching errors. This setter could perform a side effect. For example

set x(value) {
  renders++;
  _x = value
}

Then merely asserting would change the state of the program.

It’s not safe to call in the first place.

[SimenB]

Right, good point. We should just skip trying to change anything that has a setter then? Would that work?