Unable to include a literal NaN, Infinity, or -Infinity in a union type

[ljharb]

Bug Report

See the link below. I want to define a function as returning either NaN, 1, 2, or 3. I can do 1 | 2 | 3, but TS errors when I do NaN | 1 | 2 | 3, and if i do typeof NaN it allows any number.

⏯ Playground Link

Playground link with relevant code

💻 Code

const a: () => typeof NaN | 1 = () => NaN; // explicitly allows any number
const x: number = a(); // should pass, does

const b: () => 1 | 2 = () => 1; // works fine
const y: 1 = b(); // fails, as it should

const c: () => NaN | Infinity | -Infinity | 1 = () => NaN; // errors, but shouldn't
const z: 1 = c(); // should not pass, but does

🙁 Actual behavior

The c declaration errors, and the z assignment does not fail.

🙂 Expected behavior

The c declaration works, and the z assignment should fail. Why? Because NaN and the Infinities are just as much a number as 1 and 2 are, and I want to correctly type my API.

[fatcerberus]

Note #9407 (comment) - this was apparently a deliberate choice.

[ljharb]

My compelling scenario is that npm's lockfileVersion can be 1, 2, or 3, but my API (before I tried to add typings to it) returned NaN when it can't detect the version being used.

[MartinJohns]

Duplicate of #15135. Definitely not a bug.

edit: No idea why you decide to downvote my comment, when I'm just pointing you to the decision of the team.

[ljharb]

@MartinJohns thanks for the pointer :-) the downvote was for the decision.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.