microsoft / microsoft/TypeScript
TypeScript fails to detect self-reference issue in "Parameter Properties" of constructors
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Description
### 🔎 Search Terms
* private property self reference
* private property self reference constructor
* Parameter Properties constructor self reference
### 🕗 Version & Regression Information
- This is reproducible at least between v4.9.5 and nightly -- I have not tried earlier versions
### ⏯ Playground Link
https://www.typescriptlang.org/play/?target=9&module=1&ts=5.9.2#code/JYWwDg9gTgLgBAbzgKwsAdnAhgZzgfQDM4BfOQqCEOAcnQgBMBTALjCxgAsaBuAKD5YARjhhQsAY3gSANrjwBZAJ4BhOThwAhXE0QC4BuAHojcLsDwB3YDJlwJWAK45dFnI9eYA5gAoAlACEfIb2EOiiUI5S0HA+YFDAAG4cuoQsZkpgTBDERHAAvOR+eiEk+oZesQB0NVhQXjjpERheANoAusUIwSGGUEwwjlCY5jhVhD41VXUNfvylAj0GRgBUxqajcAwQTHj08JbQANYANHCcEJZMiUxQS3AgSgBijpg+aRlZOQTEhYRd9xCEzm9xIKyMfDKAlk8jgADkmJY1LCmAAPGBMdAMRSqdRaHQlQyoDDVWr1RpwZroNqdQm9MycCxVXxTGY4EELKESMKiewFODoRHwxHIjT+fjc8IQGRMKoyCC+CRVYnoHw0LA0M40ISa2gSGh+OZwIA
### 💻 Code
```ts
import { join as _f } from 'node:path';
abstract class MyClassBase {
// this will cause issue in g()!
constructor (private f: typeof _f = f) {
}
g (...args: string[]) {
return this.f(...args);
}
/* // this does not work, however
myFun (f: typeof _f = f) {
f();
}*/
}
class NewClass extends MyClassBase {
join (...args: string[]) {
this.g(...args);
}
}
const c = new NewClass();
console.log(c.join('a', 'b', 'c'));
```
### 🙁 Actual behavior
TypeScript compiler does not report any errors.
if you copy transpiled JS file and run that with Node.js, you will see
```
constructor(f = f) {
^
ReferenceError: Cannot access 'f' before initialization
```
### 🙂 Expected behavior
TypeScript clearly shows an error, similar to what would have been seen in regular function parameters
```
Parameter 'f' cannot reference itself.
```
### Additional information about the issue
The issue originated from this -- https://github.com/microsoft/vscode-js-debug-browsers/issues/19, and I just discovered that the issue in the project or TypeScript has never been fixed. If someone at Microsoft could do a favor and ping the maintainer of that repo as well, it will be much appreciated.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked Playground reproduction and the compiler’s handling of constructor parameter properties, comparing it with regular function-parameter self-reference checks. Confirm the emitted JavaScript failure and add compiler coverage so the constructor case reports an error like “Parameter 'f' cannot reference itself.”
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100