Incorrect "Property is used before its initialization" error with body-less arrow functions

[jhnns]

Bug Report

🔎 Search Terms

Property is used before its initialization
arrow function
body
property
initializer
constructor parameter
useDefineForClassFields

🕗 Version & Regression Information

• >= 3.8.3
• Also nightly

• This changed between versions 3.7.5 and 3.8.3

⏯ Playground Link

Playground link with relevant code

💻 Code

class A {
    value = 1;
}

class B {
    constructor(private a: A) {}

    // Shows "Property 'a' is used before its initialization"
    getAValue = () => this.a.value;
    
    // No error when there's a function body
    // getAValue = () => { return this.a.value };
}

useDefineForClassFields must be true

🙁 Actual behavior

With useDefineForClassFields: true there's an error when the arrow function has no body

🙂 Expected behavior

There should be no error

[akulaggarwal]

I upgraded to TS 4.7.4, problem persists.

[RyanCavanaugh]

I upgraded to TS 4.7.4, problem persists.

Can you post an example? OP's code does not error in the 4.7 Playground

[Akimotorakiyu]

@RyanCavanaugh please have a look! It's so confused!

It looks like has some trouble with compilerOptions.target in tsconfig.json, when change the target from ESNext to ES2022, this error disappear.

[MartinJohns]

Target	useDefineForClassFields	Playground	Error
ES2022	false	link	no (correct)
ES2022	true	link	no (wrong)
ESNext	false	link	no (correct)
ESNext	true	link	yes (correct)

@RyanCavanaugh On TypeScript 4.8.2, when having target set to ES2022 and enabled useDefineForClassFields there is no error, when there should be an error.

[chharvey]

Does this also address cases where the property name is used in a static field? In a static field, the object should be fully constructed so the property name should be accessible.

declare function assert(condition: boolean): asserts condition;

class Person {
    // Unexpected Error: Property 'name' is used before its initialization
    static initial_name_length = new Person().name.length;

    // fixed with an IIFE:
    static initial_name_length_fix = (() => new Person().name.length)();


    name = 'Alice';
}

assert(new Person().name === 'Alice');

assert(Person.initial_name_length === 5);

assert(Person.initial_name_length_fix === 5);

Playground