Callback parameter type is any for union of arrays

[steverep]

Bug Report

🔎 Search Terms

array, union, callback, any, contextual

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed all FAQ

⏯ Playground Link

Playground link with relevant code

💻 Code

let myCondition = true;

const arr1 = ["a", "b", "c"];
const arr2 = [1, 2, 3];

// Correctly inferred as string[] | number[]
const myArr = myCondition ? arr1 : arr2;

for (const val of myArr) {
    // val correctly inferred to string | number
  console.log(val);
}

myArr.forEach((val) => {
    // val incorrectly inferred as any
  console.log(val);
});

🙁 Actual behavior

The for..of loop correctly infers the type, but array methods (e.g. forEach, map, filter, etc.) infer the array item type to be any.

🙂 Expected behavior

Given a type which is the union of arrays and/or tuples, T1[] | T2[] | ... Tn[], array methods which take functions should infer the item parameter's type as T1[number] | T2[number] | ... Tn[number].

[steverep]

Note #37453 is nearly the same issue, but for a union of a set and an array. It was closed saying:

We're only capable of combining contextual types when the source signatures come from the same declaration

I don't claim to understand the details of that limitation, but it seems like it should not apply here since the source declaration is just the array prototype for all members of the union.

[RyanCavanaugh]

arr1.forEach((val) => {
  // val incorrectly inferred as any
  console.log(val);
});

1. It seems like you meant to write myArr instead of arr1 here?
2. If you do, it's string | number as expected
3. Please make your Playground link and your sample match.

[steverep]

Sorry, pasted the wrong code. I corrected to match the playground.

However, even if I do just correct that empty array version to use myArr, it still seems to infer any (at least in 4.9.5).

[fatcerberus]

In the Playground as posted, val is inferred string (as expected, since arr1 is string[]). If calling myArr.forEach which is what I assume you intended, then it's correctly string | number, just like the for..of version:

https://www.typescriptlang.org/play?ts=4.9.5#code/MYewdgzgLgBAtgTwMLgCYEsrvDAvDKAJwFcBTAbgFgAoG0SWAQ0MIEY8YBtAIke4BoY3AEYChwbgF0q1etBjNCAJg6dWgpYIDM0mjQD0+mChalgUADYIY6MADNSp1AogxohWwHNOkmAB8YMGI4YUcfOnB5RABBFg5EFDAMLBwAfgUWdgAuDOUZGjsQQhgACjlYADdGCxgQO3gEWMIAShgAbxoYLphDGCqa0FNzKxt7R0JSZygQNyIvf0Dg0MJOmDkQC1IAOgsQTxL+5ple7tPTgD1UmgBfPWpFVi3CwgBRRmAACxKD6tbcAD52qsur1+qNBhNhtZbA4nC4FGAEKt1psdnsfhYjgYjGdcZcbljaNQYiwnkU3p9voc8ICOtRTqDquCipDLNCxnDGK5GIjkZENttdvtDsccbiLldqNdCUA

I'm not sure where you're getting any from.

[steverep]

Ugh... finally corrected code and playground not to be arr1.foreach...

I don't know how to see the ^? inspections in the playground. They don't seem to be accessible using a screen reader. I'm simply putting that in vscode and inspecting the types there, and it's clearly any.

[fatcerberus]

It's most definitely not any. Something must be wrong with your local configuration (screenshot taken directly from your playground):

(If you can't see the screenshot - hovering over val in your playground example shows it being typed as string | number in both locations.)

[fatcerberus]

FWIW I can reproduce the any inference by setting the playground to v4.2.3 (which leads to an "implicit any" error). All later versions exhibit the expected typing.

[steverep]

So after some investigation it seems the problem only occurs when noImplicitAny is turned off. Here's a new playground with the loops trying to log val.length. The for...of loop correctly throws an error, but the forEach does not since it is typed as any.

[RyanCavanaugh]

This is the intended behavior. The forEach call used to be any with noImplicitAny off, and an error otherwise (due to the implicit any). Since it'd be a breaking change to an any to anything else, that behavior hasn't changed.

[steverep]

That's very confusing indeed...

• Nothing in the documentation for noImplicitAny suggests that inferred types would change based on the setting, only that either any will be inferred when the type cannot be determined or an error will occur.
• Why should the intended behavior be different when the array is a union of array types? If the type is string[], then forEach gives the parameter a string type regardless of noImplicitAny.

I also don't understand why fixing this would be avoided as a breaking change. If anyone is relying on any as the parameter type in this situation, then TS should expose that as a bug to fix.

Please consider altering this intention at least for version 5.

[fatcerberus]

@steverep AFAIK noImplicitAny doesn't change inferences, only whether an implicit any is an error or not. Something else really weird is going on here. In the playground link from your OP, val in the forEach call is string | number. I've double-checked this several times. In the most recent one you posted above, it's any. Regardless of toggling noImplicitAny. The code is exactly the same in both playgrounds.

@RyanCavanaugh What's going on here?

[RyanCavanaugh]

This was really done on purpose, not sure what else to say. See #42620

AFAIK noImplicitAny doesn't change inferences, only whether an implicit any is an error or not

This isn't true.

The code is exactly the same in both playgrounds.

Probably the problem is that you still have strict checked in your Playground settings? The Playground UI is confusing; unchecked means "not set", not "false".