Extend "no comparison with NaN" check to cover "Number.NaN"

[nevedaren]

Suggestion

🔍 Search Terms

x === Number.NaN
disallow comparing with Number.NaN

✅ Viability Checklist

My suggestion meets these guidelines:

• [?] This wouldn't be a breaking change in existing TypeScript/JavaScript code - this would break code containing the error
• ✅ This wouldn't change the runtime behavior of existing JavaScript code
• ✅ This could be implemented without emitting different JS based on the types of the expressions
• ✅ This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• ✅ This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Extend check added for #49962 to include Number.NaN in addition to global NaN.

📃 Motivating Example

function useVersionNumber(value: string) {
    let versionNumber = Number(value);
    if (versionNumber === Number.NaN) {
        return;
    }
    console.log(versionNumber + 1);
}

💻 Use Cases

Catching a developer error (incorrect NaN handling).

[RyanCavanaugh]

Why do people use Number.NaN ?

[nevedaren]

I don't know the exact reasoning behind it. In the cases I saw, NaN could be used instead. Maybe people wanted to be more explicit or didn't know they could just use the global NaN.

However, it is definitely used sometimes. #50626 covered majority of our cases (thanks a-tarasyuk and everyone involved!), but we also saw some rare cases of comparison with Number.NaN - e.g. I took the example attached from an existing code.

I understand it's a rare scenario, but it still might happen and the check could prevent some breakages. Would it be hard to cover this?

[RyanCavanaugh]

Not difficult, but everything costs a tiny bit of performance and it really does add up over time.

[fatcerberus]

To be fair, it seems it'd only be a perf issue because it requires a new special case - since the check already exists for the global NaN, a NaN literal type would likely solve this elegantly in the general case without need for further special cases.