Variable modified in loop supposedly "referenced directly or indirectly in its own initializer"

[jtbandes]

Bug Report

The following code produces this error on two lines:

'currentFoo' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.(7022)

This is a reduced repro case from a real code snippet. It seems like maybe the error is arising when the compiler tries to refine the types of these variables by looking at assignment expressions. But the result is unintuitive and unhelpful.

Expected behavior:

At worst, the compiler should just give currentFoo the same type as the declaration of foo, and this would be better than the current behavior.

TS playground

  let foo: {value:number} | undefined;

  function bar() {
    foo = {value: 1};

    while (true) {
      const currentFoo = foo;  // error here
      if (currentFoo) {
        const currentValue = currentFoo.value;  // error here
        foo = {value: currentValue+1};
        if (currentValue > 1) break;
      }
    } 
  }

🔎 Search Terms

implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer

Possibly related issues: #16892, #22390, #35546

🕗 Version & Regression Information

• This is the behavior in every version I tried

[RyanCavanaugh]

currentValue is referenced in its own initializer during the second iteration of the loop.

[jtbandes]

It is? How so?

Also note that adding a type annotation to currentValue on line 9 fixes the problem:

TS playground

  let foo: {value:number} | undefined;

  function bar() {
    foo = {value: 1};

    while (true) {
      const currentFoo = foo; // no error
      if (currentFoo) {
        const currentValue: number = currentFoo.value; // type annotation added here -- no error
        foo = {value: currentValue+1};
        if (currentValue > 1) break;
      }
    } 
  }

[RyanCavanaugh]

It is? How so?

Because the kinds of values that can inhabit foo are determined what can possibly be in currentValue, which in turn picks up a value from currentFoo at the top of the loop, which in turn depends on what foo might be

[jtbandes]

Would it be reasonable to automatically fall back to the type that foo had before it acquired the dependence on currentValue?

Even sticking to the very first type foo is given ({value:number} | undefined) would be enough to make the definition of currentFoo typecheck without causing problems downstream. You would also have to apply the same logic to currentValue, inferring its type without any refinement of currentFoo beyond the first time it's used.

I know my terminology above isn't very precise, but my mental type-checking thought process goes:

• foo is a {value:number}|undefined
• currentFoo is initially {value:number}|undefined, and inside if(currentFoo) it's known to be {value:number}
• therefore currentFoo.value is a number
• therefore currentValue is a number ✅

Basically, I think that this case can be solved by applying less advanced type inference than what the compiler is currently trying to do...

[RyanCavanaugh]

Silently falling back to the declared type is going to just make things more confusing in the cases where a circularity exists, because then it will often look like narrowing is broken in much more subtle ways.

[jtbandes]

I can see that being confusing, but only in the case that the result has a type error. Is it possible to try the fallback and only use it if it works?

[RyanCavanaugh]

"Do this thing if it happens to make the program pass, otherwise do this other thing" logic is O(2^n) on the nesting level of your code, which isn't conducive to good compile times.

[jtbandes]

I guess the best I can hope for then is a clearer error message :)

[garyo]

#43708 is considered a dup of this bug, and I don't want to beat a dead horse (I do have a workaround) but in principle it seems like ts ought to be able to figure that case out (this is the minimal repro from that bug report):

declare function foo(x: string | undefined): Promise<string>

async () => {
  let bar: string | undefined = undefined;
  do {
    const baz = await foo(bar); // ERROR HERE
    bar = baz
  } while (bar)
}

In this case, the return type of foo is always Promise<string> so in the const baz line, the type of baz is definitely known to be string, right? (Independent of the current type of bar, which does switch from undefined to string.) So it seems to me that there should be no recursion there.

[jtbandes]

Your example seems a little different, because it involves async/await: if I change Promise<string> to string and remove the await, it works. I agree it feels like the same kind of issue, but my original example doesn't involve async. I wonder how the async ends up triggering the problem...

[bvallee-thefork]

Facing the same issue.

Same as with #43708, it works when removing the await and Promise<>.

The temporary workaround we're using: explicitly casting the "circular" value.

Example (with the indirectly in its own initializer error):

declare function myQuery(input: { lastId: number | undefined }): Promise<{ entities: number[] }>;

async function myFunc(): Promise<void> {
  let lastId: number | undefined = undefined;

  while (true) {
    const { entities } = await myQuery({
        lastId,
    });

    lastId = entities[entities.length - 1];
  } 
}

Workaround (no error):

declare function myQuery(input: { lastId: number | undefined }): Promise<{ entities: number[] }>;

async function myFunc(): Promise<void> {
  let lastId: number | undefined = undefined;

  while (true) {
    const { entities } = await myQuery({
        lastId,
    });

    lastId = entities[entities.length - 1] as number | undefined;
  } 
}