Typescript: Arrow functions don't work in super calls

[markboyall]

Typescript version: 2.1.4

Playground: https://www.typescriptlang.org/play/index.html#src=class%20C%20%7B%0D%0A%09constructor(public%20f%3A%20%7B%20()%3A%20string%20%7D)%20%7B%0D%0A%0D%0A%09%7D%0D%0A%0D%0A%09%0D%0A%7D%0D%0A%0D%0Aclass%20B%20extends%20C%20%7B%0D%0A%09constructor()%20%7B%0D%0A%09%09super(()%20%3D%3E%20%7B%20return%20this.g()%3B%20%7D)%0D%0A%09%7D%0D%0A%0D%0A%09private%20g()%20%7B%0D%0A%09%09return%20%22hello%22%3B%0D%0A%09%7D%0D%0A%7D%0D%0A%0D%0Aalert(new%20B().f())%3B

Source:

class C {
    constructor(public f: { (): string }) {
    }   
}

class B extends C {
    constructor() {
        super(() => { return this.g(); })
    }

    private g() {
        return "hello";
    }
}

alert(new B().f());

Expected result: "Hello" is alerted.
Actual result: calling f() fails with referenceerror.

The problem is that arrow functions are bound by the use of _this. Unfortunately, when there is a super call with no other this references, Typescript skips emitting the _this variable. _this doesn't exist so when the function is called, it fails.

If somehow this is the intended behaviour, the compiler should error on this references in super calls.

[timocov]

I guess there should be an error, because super must be called before accessing this.
Just change constructor of B to code below and you will get error (the code is equal)

super(this.g.bind(this));

[RyanCavanaugh]

This is basically working as intended because we have no way to know that the base class constructor invokes the function immediately. If instead the base class constructor stored a reference to the closure and then invoked it later (for example, maybe this function is an event handler), no error would occur and the code is fine.

[markboyall]

I don't understand? The base class constructor does do exactly that in my sample- it stores the reference and then it is called later. But the error still occurs.

[RyanCavanaugh]

Sorry, misread. You're right - we should have emitted var _this = this; above the super call

[markboyall]

Just for reference, it seems to me that even if the base class invokes the callback immediately, that should be safe as long as it doesn't return a new value for this?

[mhegazy]

Fix should be in [email protected] or later.