You actually can get Infinity type if you're clever and other fun floating point oddities

As explained in #15135 and #15351, the Typescript team doesn't intend to support NaN and Infinity literals, due to complexity. Explicitly typing something as Infinity or -Infinity gives the error:

'Infinity' refers to a value, but is being used as a type here.

As it turns out, you actually can get the Infinity type fairly easily. Just use a really large number that parses to infinity as your type, like 1e999. -1e999 gives you negative Infinity. Fortunately, I don't believe you can get NaN from just parsing a double precision literal other than NaN itself. 0/0 is the simplest way to get NaN, but that's 2 literals and a divide.

There are a number of other floating point literal peculiarities resulting from round-off as well. I fully expect the Typescript team to "won't fix" them:

type DoesntExtend = 1.000000000000001 extends 1 ? 'yes' : 'no' // Resolves to 'no'
type Extends      = 1.0000000000000001 extends 1 ? 'yes' : 'no' // Resolves to 'yes'

Here's another.

type NegativeZero = -0 extends 0 ? 'yes' : 'no'; // Resolves to 'yes'

In this case, -0 and 0 are actually distinct numbers with different bit patterns. Both Chrome and Firefox dev consoles correctly echo -0 as -0 rather than 0. However, the IEEE-754 standard defines that 0 === -0, so I'm not surprised Typescript thinks they're the same type. The intent for this distinction is to convey the idea of limits as a numeric representation (not to mention floating point is sign-magnitude anyways, so why not embrace it). For example, 1/-Infinity and -8*0 both result in -0 rather than 0. For a type system, I'm not sure that you strictly need or want to maintain this distinction.

TypeScript Version: Repro'd on playground

Search Terms:
Infinity, NaN
Code

type Illegal = Infinity; // Doesn't compile
type Inf = 1e999; // Resolves to Infinity
type NegativeInf = -1e999; // Resolves to -Infinity

Expected behavior:

Actual behavior:

Playground Link:
Here

Related Issues:

Start with the linked TypeScript Playground reproduction and read related issues #15135 and #15351. The issue does not define expected behavior or name compiler files or tests; a complete contribution would first need a settled decision about Infinity, NaN, floating-point rounding, and negative zero handling.