expect `toBe` has misleading error message

[ggarek]

Do you want to request a feature or report a bug?
Feature (it is more like a fix)

What is the current behavior?
Sometimes developer may want to check for identity.
Currently, expect.toBe does this.
But the error message is misleading:

Compared values have no visual difference. Looks like you wanted to test for object/array equality with strict `toBe` matcher. You probably need to use `toEqual` instead.

Because the message suggests to change toBe -> toEqual. So new developer may make this change which may lead to a bug in the project (or deoptimization).

I also think, that toBe and toEqual do not communicate the difference between each other clearly.

Suggestion

1. Maybe change name toBe -> toBeExact or toShallowEqual.
2. Maybe change message You probably need to use 'toEqual' instead. -> If you want to check for deep equality, pease, use 'toEqual' instead.
3. Maybe change toEqual -> toDeepEqual

[SimenB]

Do you have an example of code leading to that message?

[ggarek]

Sure

file

test('identity check', () => {
  const value = {};
  const identity = x => Object.assign({}, x); // Wrong implementation
  expect(identity(value)).toBe(value);
});

output

  FAIL  src/test.js
  ● identity check

    expect(received).toBe(expected) // Object.is equality

    Expected value to be:
      {}
    Received:
      {}

    Difference:

    Compared values have no visual difference. Looks like you wanted to test for object/array equality with strict `toBe` matcher. You probably need to use `toEqual` instead.

      2 |   const value = {};
      3 |   const identity = x => Object.assign({}, x); // Wrong implementation
    > 4 |   expect(identity(value)).toBe(value);
      5 | });
      6 |

      at Object.<anonymous> (src/test.js:4:27)

jest v22.4.3

[rickhanlonii]

Ah @ggarek, so when toBe is semantically the correct matcher for your code, we're implying it should be changed

[SimenB]

Jest has no way of knowing that it is semantically correct for your program for them to be equal, but not referentially the same.

But maybe we could change the message to contain something about the fact is toBe is Object.is, while toEqual is deep equality?

[rickhanlonii]

How about:

[SimenB]

I like it

[rickhanlonii]

@ggarek what do you think? Do you want to submit the PR?

[ggarek]

Ah @ggarek, so when toBe is semantically the correct matcher for your code, we're implying it should be changed

Exactly. So, changing message will help here, i think.

I will submit a PR. Thanks, @rickhanlonii @SimenB

[SimenB]

#6122