Error TS2729 should occur inside arrow function

[steffen-4s1]

🔎 Search Terms

• ts2729
• used before its initialization
• arrow function in fields
• useDefineForClassFields

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about >= 5

⏯ Playground Link

https://www.typescriptlang.org/play/?useDefineForClassFields=true&target=11#code/MYGwhgzhAEBiD29oG8BQ1oAcCuAjEAlsNAGYECmIAJgILQC80A2gLoB0AtmJgBQ8CUDAHwp0GaABcAFgQhsI5AE4A3IuTZV4AsQF9+qMTnxFSFagCEGkmXIUq1GrfsN5CxYPAB2ECYuzAJeEUeTEUCZTAJcmg7VWByAC4UaE0kgWEY3wJPAHNoPRQdVB0gA

💻 Code

class Foo {
 public fieldA = [].map(() => {
 // access service inside arrow function shouldn't be work with useDefineForClassFields = true
 this.service.do()
 // ^^^^^^^ expected error ts(2729)
 })

 public fieldB = this.service.do()
 // ^^^^^^^ Property 'service' is used before its initialization. (ts(2729))

 public constructor(private service: { do: () => string }) {}
}

Just for sake of completeness:

JavaScript output with useDefineForClassFields = true

class Foo {
 service;

 fieldA = [].map(() => {
 this.service.do();
 });

 fieldB = this.service.do();

 constructor(service) {
 this.service = service;
 }
}

JavaScript output with useDefineForClassFields = false

class Foo {
 constructor(service) {
 this.service = service;

 this.fieldA = [].map(() => {
 this.service.do();
 });

 this.fieldB = this.service.do();
 }
}

🙁 Actual behavior

Error Property 'service' is used before its initialization.(2729) only occurs for fieldB initialization.

🙂 Expected behavior

Error Property 'service' is used before its initialization.(2729) should also occur for fieldA initialization inside the arrow function.

Additional information about the issue

With useDefineForClassFields = true (which is the default) the generated JavaScript code in my example won't work, because service is accessed even though it is still undefined.

If I set the setting to false, as is done automatically for earlier targets, everything works. However, I would like to avoid using the old format for classes.

[MartinJohns]

This is working as intended. The compiler doesn't know when your arrow function is invoked, and it's a reasonable assumption it will be invoked after your variable is set.

This would be improved with #58729, allowing the compiler to know that the arrow function passed to Array.map() will be invoked immediately.

[steffen-4s1]

Thank you for your feedback and clarification.