Ambient public fields initialized in parent class are blocked at compile-time

[robpalme]

TypeScript Version: 3.8.0-dev.20191123

Search Terms: ambient declare public class fields parent initializer useDefineForClassFields

Code:

// @useDefineForClassFields: true
class Parent {
    a: any;
    constructor(arg: any) {
        this.a = arg;
    }
}
class Child extends Parent {
    declare a: number;
    constructor(arg: number) {
        super(arg);
        console.log(this.a);  // Property 'a' is used before being assigned. (2565)
    }
}

Expected behavior:

There should be no compile-time error, because this is legitimate ES(next) code. The intent of declare <field> is merely to specialize the type, not to impose additional initialization constraints.

Actual behavior:

The usage of this.a causes the compiler error Property 'a' is used before being assigned. (2565).

The emitted code is good. The only problem is the compile-time error.

Playground Link:

• Minimal repro
• Example from the 3.7 release announcement

Related Issues: #34972

The main use-case for ambient public fields is to specialize parent class fields. It is common for parent class fields to only be initialized in the parent constructor, invoked via super(). Due to this bug, this pattern does not work today, forcing the use of a workaround such as using @ts-ignore or redundant re-initialization of the property in the child constructor to appease the compiler.

cc: @sandersn

[fatcerberus]

Interestingly, the error is correct, but only because the canonical (i.e. untransformed) emit is apparently wrong. With useDefineForClassFields: true and target: "esnext", the emitted code is:

"use strict";
// @useDefineForClassFields: true
// @target: esnext
class Parent {
    a;
    constructor(arg) {
        this.a = arg;
    }
}
class Child extends Parent {
    a;
    constructor(arg) {
        super(arg);
        console.log(this.a); // Property 'a' is used before being assigned. (2565)
    }
}
new Child(3); // prints undefined

Child#a is not actually ambient. With that emit, constructing a Child prints undefined, regardless of the value passed to the constructor. It really is used before being assigned! 😄

🏞 Playground Link

[fatcerberus]

It seems the emit was fixed post-3.7, as the ambient field isn't emitted after switching the Playground to Nightly and the code then works correctly. This does perhaps shed some light on how the error in question came to be, though.

[robpalme]

You're right @fatcerberus that the original issue #34972 has now been fixed on master. Upon testing that fix I then discovered this compile-time error, so raised it here.

[fatcerberus]

It's just very interesting that the error message is correct for the original (incorrect) behavior.

[robpalme]

Good point. I guess it's obvious in retrospect, because the fix only changed the transformer. There was no change to the checker.

[robpalme]

Thanks for scheduling this @RyanCavanaugh.

This feature (declare with class fields) was introduced in 3.7 and the showcase example code from the 3.7 release announcement still doesn't work due to this issue. It is currently scheduled for 3.8 - I wonder if this qualifies for patching 3.7?