Array.filter accepts any

TypeScript Version: 2.7.0-dev.20171114

Code

function f(xs: number[]) {
    xs.every(x => x + 1); // Error
    xs.some(x => x + 1); // Error
    xs.find(x => x + 1); // Error
    xs.filter(x => x + 1); // No error?
}

Expected behavior:

All are errors.

Actual behavior:

The callback passed to filter is not type checked.

This was changed in #7779. But it seems like if developers want that behavior we should just have a flag to make boolean behave like any (and enable it by default for jsconfig.json) instead of trying to avoid using the boolean type ever.

In --strictNullChecks we could also check that the value is actually potentially truthy/falsy, so string and Node | undefined would work but not just Node.

It's not really possible for a lint rule to check this kind of situation because the signature of the function doesn't indicate that the value is meant to be a boolean. So it seems like the correct solution is to write declaration files with specific types and allow users to have more loose behavior.

[thorn0]

The flag sounds like an interesting idea. Would it affect all the instances of boolean or only those in the return type position? Or would there be some special type for signatures like filter, an alias for boolean in the 'normal' mode and for any if the flag is set?

[aj-r]

I'm a little uneasy about a global flag that changes the behaviour of all boolean types. Having it only affect boolean callback return values is a bit better, but I feel like it should be up to the function itself to decide whether it wants a real boolean (true/false) value.

Maybe a new type (BooleanLike? TruthyFalsy?) would do the trick? It would indicate that the value is meant to be a boolean, but would allow other truthy/falsy values based on the rules that @Andy-MS described. It means that libraries will need to update to use this new type, but I think that's a good thing, because it allows each library to choose whether they want that functionality or not. And we won't be breaking anything for them in the meantime.

Re: @thorn0: I don't think we want to allow any value if this flag is set (or BooleanLike is used). Some return types like void should not be allowed.

[mhegazy]

Duplicate of #19456

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.