Uninitialized instance properties can be accessed via initializers

[igrep]

🔎 Search Terms

• "constructor instance variable undefined"
• "instance property undefined constructor uninitialized"

🕗 Version & Regression Information

> npx tsc --version
Version 5.4.3

⏯ Playground Link

https://www.typescriptlang.org/play?noUncheckedIndexedAccess=true&ts=5.5.0-dev.20240328#code/MYGwhgzhAECiAeYC2AHEBTaBvAUNaYAXNBAC4BOAlgHYDmA3HtAEbQC80pAFpRAHQBXCOgCCACgCUjfMAD21MuQHBSs8mKIkKNWhOxN83XnzDsC06AF8cTIaMnFFO-fnzl0pAeWqce-MHyqAKooKOjkAMKQ6JIW1tY4cgqyGHwgsrRi1OgA7nCIqBhiAOToBWjoxRJ8zFJAA

💻 Code

tsconfig.json:

{
  "compilerOptions": {
    "target": "es2016",
    "module": "commonjs",
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "strict": true
  }
}

example.ts:

class Example {
  a: string;
  b = this.useA();
  constructor(a: string) {
    this.a = a;
  }

  useA(): string {
    return this.a.toUpperCase();
  }
}

console.log(new Example('example').b);

🙁 Actual behavior

Produces no compile error, and generates a code that throws a runtime error:

TypeError: Cannot read properties of undefined (reading 'toUpperCase')

🙂 Expected behavior

Produces a compile error similar to "Property 'a' is used before its initialization."

Additional information about the issue

I'm not sure whether this may be a issue that can be fixed or just keep it as is "by design". I haven't found any other issues similar to this one here, at least with the search terms I tried.

As you know, we can easily modify the example case like below to avoid the error:

class Example {
  b = this.a;
  constructor(public a: string) {
    this.a = a;
  }

  useA(): string {
    return this.a.toUpperCase();
  }
}

console.log(new Example('example').b);

But we can't adopt this workaround when the property is a hard-private one:

class Example {
  #privA: string;
  #privB = this.useA();
  constructor(privA: string) {
    this.#privA = privA;
  }

  useA(): string {
    return this.#privA.toUpperCase();
  }

  get a(): string {
    return this.#privA;
  }

  get b(): string {
    return this.#privB;
  }
}

console.log(new Example('example').b);

Actually I found this problem when using a hard-private variable.

[MartinJohns]

Duplicate of #38205 / #11006.

[igrep]

Sorry. I failed to search the existing issues well.