JavaScript class property becomes `any` without warning when used in a function that is assigned to itself about typescript HOT 5 OPEN

Not a bug; this behavior follows from the JS inference rules.

The implied feature request here is to issue more circularity errors.

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It actually has nothing to do with the self-assignment; the core of the issue is that you’re initializing it to null, which causes TS to infer any for the type of that property (the compiler figures you probably don’t want it to stay as null so that would be a useless inference, and evolving types for properties aren’t a thing).

Workaround: Explicitly type the property as number | null. I’m not sure how to do that with JSDoc; maybe a @type tag?

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Hmm I'm not entirely sure about that, but you might be right.
For example, if you replace that line with this, the issue is also solved:

this.maybeNum = 1;

(playground)

Since Math.max() is known to always return a number, shouldn't TypeScript be able to type the property implicitly?
For instance, this also fixes the issue:

this.maybeNum = Math.max(1);

(playground)

So that seems to suggest that this is a bug after all.

I'm aware the fix is to explicitly give it a type. However, since I have TypeScript configured with noImplicitAny, I would expect TypeScript to warn me that this property has an any type. But now that I think about it, this is appears to be a separate issue.

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Since Math.max() is known to always return a number, shouldn't TypeScript be able to type the property implicitly?

But you're passing this.maybeNum in, so at that point the compiler must already know the type of maybeNum. The compiler doesn't do type checks in two phases.

Besides that, the type number would clearly be wrong, as you assign null do it in the constructor. So the type would have to be number | null.

As @fatcerberus suggested, you can do this:

/** @type {number | null} */
this.maybeNum =  null;

from typescript.

The compiler doesn't do type checks in two phases.

But it doesn't need two phases though!

If you turn it into this, it also infers the type correctly:

const x = Math.max(this.maybeNum);
this.maybeNum = x;

(playground)

So this is clearly a weird edge case and I'm pretty certain this is a bug :)
If not, then at least this should be considered a feature request.

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