strictNullChecks safeguards against null and undefined, but not NaN

[MarvinHannott]

Code

// simple convertion
let n: number = Number("not-a-number") // could be NaN

// function that returns NaN
function couldBeNaN(x: any): number {
     if(typeof x === "number"){
            return x;
     }
     return NaN
}

n = couldBeNaN("not-a-number") // will return NaN 

Expected behavior:
A safeguard against NaN as there is for null and undefined.

// simple convertion
let n: number | NaN = Number("not-a-number") // enforced check

// function that returns NaN
function couldBeNaN(x: any): number | NaN {
     if(typeof x === "number"){
            return x;
     }
     return NaN
}

n = couldBeNaN("not-a-number") // will return NaN, but it's okay 

Hence NaN isn't a type in TypeScript, this isn't possible.

Actual behavior:
NaN passes as a regular number although it is the direct opposite (yes, I know it is defined as such in the ECMAScript-standard, but nonetheless, it almost always hits you unexpected).

[RyanCavanaugh]

Strict null checks isn't about NaN because NaN isn't null (or undefined). If you'd like to write a suggestion proposal, please follow the guidelines for that.

[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.