typeof this stops working after for..in or for..of: "Cannot find name 'this'."

[Yonom]

Bug Report

After a for..in or for..of statement, I can no longer access typeof this in class methods. A compile time error appears saying "Cannot find name 'this'."

🔎 Search Terms

Cannot find name 'this'. typeof this class for in for of

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about typeof, classes, typeof this

⏯ Playground Link

Playground link with relevant code

💻 Code

class Tests {
    test1() { // OK
        type Test = typeof this;
    }

    test2() { // OK
        for (;;) {}
        type Test = typeof this;
    }

    test3() { // NOK: expected no compile errors
        for (const dummy in []) {}
        type Test = typeof this; // Error 2304: Cannot find name 'this'.
    }

    test4() { // NOK: expected no compile errors
        for (const dummy of []) {}
        type Test = typeof this;  // Error 2304: Cannot find name 'this'.
    }
}

🙁 Actual behavior

The test cases test3 and test4 show compile errors.

🙂 Expected behavior

All test cases should pass without error.

[andrewbranch]

Congratulations, this is officially the weirdest bug I’ve seen today

Uh, also, all of these were errors until 4.4?!

[fatcerberus]

In the meantime, for the record: typeof this and this are the same type.

[paranoiacblack]

Something interesting to note here is that it compiles fine if there are any statements within the for..in or for..of loop.

[Zzzen]

test2 is an infinite loop, so anything could happen.

[Zzzen]

In the meantime, for the record: typeof this and this are the same type.

Actually, they are slightly different: this always refers to the current scope of it while typeof this will use its flow type.

class Base {
    f() {
        if (this instanceof D) {
            this.g();
            (this as this).g();  // Error: Property 'g' does not exist on type 'Base'.(2339)
            (this as typeof this).g();
        }
    }
}

class D {
    g() {}
}

[ahejlsberg]

Looks to be an effect of #43898.