False positive for TS2729 (use before initialization) with target = ESNext

[slavafomin]

Bug Report

Hello!

I've encountered this problem after updating my project to the latest TypeScript 4.3.2.

The bug looks similar to #37573.

💻 Code

script.ts

class Foo {

  constructor(private value) {
  }

  anotherValue = this.value;

}

tsconfig.json

{
  "compilerOptions": {
    "target": "ESNext"
  }
}

This happens only when using ESNext as a target. Using ES2020 works correctly. Could you also elaborate on the difference of these two settings?

🙁 Actual behavior

I'm getting an error: error TS2729: Property 'value' is used before its initialization.

🙂 Expected behavior

The code should compile just fine.

[whzx5byb]

My project also use esnext as the target but I can not reproduce the error.

[entropitor]

I have the same issue as @slavafomin when I upgraded to 4.3 in a project targetting esnext, es2020 solves it indeed!

[whzx5byb]

After some investigation, I think this is an intended behavior.
Consider the following code:

class A {
  foo = this.bar;
  constructor(private bar: any) {}
}

A esnext target will compile it to the native class field.
And this is actually an error because bar is assigned to foo when it is not initialized.

// target: esnext
class A {
    bar;
    foo = this.bar;
    constructor(bar) {
        this.bar = bar;
    }
}

The es2020 target will use Object.defineProperty to compile the code, and rearrange the assignment order.

class A {
    constructor(bar) {
        Object.defineProperty(this, "bar", {
            enumerable: true,
            configurable: true,
            writable: true,
            value: bar
        });
        Object.defineProperty(this, "foo", {
            enumerable: true,
            configurable: true,
            writable: true,
            value: this.bar
        });
    }
}

=========
PS:

My project also use esnext as the target but I can not reproduce the error.

The reason is that I am using "useDefineForClassFields": false in my project. It will produce the same output regardless of what the target is.

class A {
    constructor(bar) {
        this.bar = bar;
        this.foo = this.bar;
    }
}

[RyanCavanaugh]

@whzx5byb has it exactly right. The error is being issued exactly when a problem will occur.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[sirreal]

This seems to be the PR that introduced the (expected) change in compiler behavior: #43194

[Bilge]

The es2020 target will use Object.defineProperty to compile the code, and rearrange the assignment order.

Except that, in your example, the assignment order is still exactly the same (as it should be).

[Artoria2e5]

Can reproduce with playground, although now with es2020 the transpile does not have Object.defineProperty (probably because the default for useDefineForClassFields is now target-dependent -- weird compromise):

class Foo {
    constructor(value) {
        this.value = value;
        this.anotherValue = this.value;
    }
}
export {};

Bumped into this issue in https://github.com/homebridge/homebridge-plugin-template/blob/fe5bb73eee3efa007f137b9268e9a6c59b88d9ef/src/platform.ts#L12. There we have api being reported undefined.

(ES2022 is out, so now we get people who's not specifying ESNext bumping into this.)