Misleading Error Message with == and valueOf

[SteveBeeblebrox]

Bug Report

🔎 Search Terms

always false, valueOf, equality

🕗 Version & Regression Information

• This changed between versions 4.7.4 and 4.8.4 (and still happens on nightly)

⏯ Playground Link

Playground link with relevant code

💻 Code

console.clear()
console.log('foo' == {valueOf() {return 'foo'}}) // true

🙁 Actual behavior

The above code snippet produces an error on the == condition: This condition will always return 'false' since JavaScript compares objects by reference, not value.(2839) which would lead the user to believe that console.log(...) would always print false. In reality, the line prints true. The object on the right is coerced to a string using the valueOf() method so the comparison becomes 'foo' == 'foo' which is true, but TypeScript claims otherwise.

🙂 Expected behavior

Even if TypeScript cannot determine that this condition would be always true given the usage of valueOf, I would expect the error message to say something more like ...may always return false...
instead of the more certain ...will always return false... so that the user is not told that something will certainly happen when it may not.

[MartinJohns]

I'd call this a duplicate of #2361. valueOf() is just not really supported by the type system.

[SteveBeeblebrox]

I don't expect TypeScript to understand the valueOf(), but it would be nice if the error message wording didn't contradict runtime behavior.

[fatcerberus]

What do you propose? The error message is true if you name the method literally anything else, so you’re de facto asking typescript to understand the semantics of valueOf, which it currently doesn’t.

[RyanCavanaugh]

It's a waste of time and effort to make the message less clear for the sake of satisfying cases that can only be hit through specific construction.