Bug Report

🔎 Search Terms

• Infinity
• NaN
• numeric
• literal

🕗 Version & Regression Information

• Tested in: TypeScript 4.1.5 and TypeScript 4.3.0‑dev
• I was unable to test this on prior versions because Number, enum, and boolean literal types #9407 was implemented in TypeScript 2.0.

💻 Code

// @declaration
// @showEmit
// @showEmittedFile: index.d.ts

// @filename: index.ts
export const PositiveInfinity: 1e1_000_000 = 1/0 as any;
export const NegativeInfinity: -1e1_000_000 = -1/0 as any;

⏯ Playground Link

Workbench Repro

🙁 Actual behavior

TypeScript emits Infinity and ‑Infinity in a type position, which are intentionally invalid according to #9407 (comment):

// index.d.ts
export const PositiveInfinity: Infinity;
export const NegativeInfinity: -Infinity;

🙂 Expected behavior

The Infinity and ‑Infinity values are valid in a type position, so that the generated .d.ts file is valid.

Also, it’d be nice to support literal NaNs, which would allow for Number.isNaN to be typed as:

interface NumberConstructor {
	isNaN(number: unknown): number is NaN;
}

Related issues

• Add a --strictNaNChecks option, and a NaN / integer / float type to avoid runtime NaN errors #28682
• You actually can get Infinity type if you're clever and other fun floating point oddities #31752
• Allow Inifinity and -Infinity as number literal types #32277
• Infinities as number literal types #56272