Infer parameter as number when using Number.is*

[til-schneider]

When using the Number.is* (Number.isFinite, Number.isInteger, Number.isNaN and Number.isSafeInteger), the parameter's type should be inferred as number.

Example:

function foo(value: number | null | undefined) {
    if (Number.isFinite(value)) {
        bar(value)
    }
}

function bar (value: number) {
    ...
}

[typescript-bot]

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[fatcerberus]

The reason this isn't the case is because:

• TS playground

function isFinite(x: unknown): x is number
{
    return Number.isFinite(x);
}

let foo: number | string = Infinity as number | string;
if (!isFinite(foo)) {
    foo;  // string?!
}

Type predicates work in both directions.

[til-schneider]

@fatcerberus: This would be true for the global isFinite, but Number.isFinite returns false if you pass a string.

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isFinite

In comparison to the global isFinite() function, this method doesn't first convert the parameter to a number. This means only values of the type number and are finite return true.

Example:

console.log(Number.isFinite('12'))   // false

[MartinJohns]

@til-schneider Notice the negation operator before.

And see #39090 (comment).

[til-schneider]

OK, I see. That's unfortunate...

So the only correct inference would be isNaN(number: unknown): number is NaN;?

[til-schneider]

No, this does't work either. Only number is typeof NaN would work, but typeof NaN is number again...

[MartinJohns]

That would be the case if NaN would be a valid type.

[fatcerberus]

@til-schneider Notice that I wasn’t using the built-in global isFinite in my example, but one I implemented manually exactly to illustrate the issue.