No error when initializing a class field defined as a read-only getter in base class

[trusktr]

TypeScript Version: 3.6.4

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Code

class Base {
  get foo() {
    return 5
  }
}

class Child extends Base {
  foo = 10
}

new Child // runtime error!

Expected behavior:

Type error during compile

Actual behavior:

No error until runtime.

Playground Link:

playground

Related Issues:

Might be duplicate of #13347, but not sure.

[AviVahl]

Checked this snippet of code directly in Chrome, and there's no runtime error when evaluating new Child. I get a new Child instance with a foo field === 10, and it is assignable (unlike instances of new Base).
do you mean you get a runtime error if you transpile this using typescript (to es5) and then run?

I believe this issue has to do with: #27644

[fatcerberus]

I don't know how TypeScript is supposed to handle this, but classfields in ES use [[Define]] (think Object.defineProperty) which bypasses the setter in the base class, so this is legal JS; that's probably why you're not getting a runtime error in Chrome @AviVahl.

[trusktr]

@AviVahl Yeah, exactly, if TS compiles to the form

constructor() {
  this.foo = 10
}

then it doesn't work. This can happen with either es5 target, or the new "useDefineForClassFields": false option (I haven't tested it with the latter).

[sandersn]

This is an error with useDefineForClassFields: true, which will be the default target for target: "esnext" when class fields reach stage 4. That's because it's quite confusing, even though it's legal [stage 3] JS. It could always be an error, but this is a problem that real code rarely encounters. When I looked at our user test suite, the only examples I found were in old, pre-readonly code, and I believe Google and Bloomberg found the same to be true on their internal code bases.

This would be a nice feature but not required, and fairly easy to implement since the error is already there, so I'm going to move this into the backlog and put the Help Wanted label on it. It's a good feature if you want to focus on identifying breaks in the ecosystem.

Edit: Well, I thought about it a bit more and decided to at least create the PR. There are lots of breaks in the test suite because we test exactly this weird case a lot, so I'll put it up tomorrow after verifying that they're correct.

[trusktr]

@sandersn Thanks for the fix! Btw, I see the error message is

'foo' is defined as an accessor in class 'Base', but is overridden here in 'Child' as an instance property.

in 4-beta playground even if useDefineForClassFields is disabled and I have a setter. Isn't that code legal when useDefineForClassFields is false?

That is amazing that when useDefineForClassFields is set to true that there is a type error on accessor overrides! Very nice in helping people avoid problems.

[sandersn]

We decided to disallow properties overriding accessors in 4.0 for everyone, so that when class fields finally do move to stage 4, any code that could get tripped up by the [[Set]] -> [[Define]] migration would have time to get fixed.

[ntucker]

what's wrong with this if the Child class marks it as readonly?

[sandersn]

There's still some kind of initialisation, and whether that affects base accessor depends if you are compiling with [[Set]] (classic TS) or [[Define]] (ESNext) semantics. For example:

class Base {
  get foo() {
    return 5
  }
}

class Child extends Base {
  readonly foo = 10
}

Still crashes with [[Set]] semantics, because the emitted code has a this.foo = 10:

class Base {
    get foo() {
        return 5;
    }
}
class Child extends Base {
    constructor() {
        super(...arguments);
        this.foo = 10;
    }
}

[isral]

@sandersn
I have setter only property
But the code that get the property is not considered compile error (return undefined at runtime).
Is this related to this bug? Or should I issue a new bug?
Typescript 4.3.2 (VS Code) 4.3.5 (Command Prompt).

[sandersn]

@isral Please do open a new bug. It's a lot easier with an example to look at.