Type narrowing fails when using comma operator for function parameter

[dummdidumm]

TypeScript Version: 4.0.2
Search Terms: comma operator flow narrow type function

I encountered a case where type narrowing fails when using the comma operator and passing the result to a function which does type narrowing.

Code

const otherValue = () => true;
const value : number | string = null as any;

function isNumber(obj: any): obj is number {
    return true; // method implementation irrelevant
}

// Good case - works
if (typeof (otherValue(), value) === 'number') {
    const b = value; // number
}

// Bad case - fails
if (isNumber((otherValue(), value))) {
    const b = value; // string | number , but should be number
}

Expected behavior:
I would expect the second case to also narrow down the type to number

Actual behavior:
Type is string | number, it was not narrowed.

Playground Link:
https://www.typescriptlang.org/play?#code/MYewdgzgLgBCUAsCmAnAagQwDYFckwF4YAKASkID4YoU8BuAKFElgDds8YAuGMHAWwBGqGAB8Y0FAEswAc0K8cWLDAwRVYAJ6MGAMxxhgUKeBhSIAOQHCUxEIIBWPDFtI97Ds+r5CRAbwYYIJgUJCgcFDBqWiQ6GAB6eJh+MIQQABMzfgAHLCQUsCgMY1MpFFC89kKGAF8GBildEihNbKQQJrtEVExcJDIAGhh2PvICcZgAch8bSfIA4JhmaBhBBRH6BKSZ1Fr6xpJzK19bLuR0Dn7SIY2kUnuYBeDl2DWiW7jEiRoZeXEdlC1IA

Related Issues:
Sounds a little related: #35484

[GrzegorzKazana]

Edit: please ignore, i was wrong

This is an interesting issue - I wonder if narrowing down the comma expression in any case is actually the correct behavior.
Doing so is not safe, since we may trigger a side effect from inside of it.
(Obviously the question is why anyone would do such thing, but still)

For example:

const isNumber = (x: unknown): x is number => typeof x === 'number'
let thing = 42 as number | string;

const doSth = () => {
    thing = '100% evil stuff'
}

if (typeof (doSth(), thing) === 'number') {
    thing; // number
}

In this case narrowing to number is a mistake - since doSth changed thing to string.

So I think there are two topics:

• one explained by @dummdidumm, which is inconsistency when using typeof guard and custom type-guard - why does only one of them actually narrows down the type
• should comma expressions be even narrowed down?

[MartinJohns]

In this case narrowing to number is a mistake - since doSth changed thing to string.

It's not a mistake. The comma operator evaluates each of its operands from left to right. Sure, it changes the value to a string inside the function, but that just results in the comparison to be false.

[GrzegorzKazana]

Ahh, my bad. Thanks for correcting me @MartinJohns 😅