Uninitialized instance properties can be accessed via initializers #57984
Description
🔎 Search Terms
- "constructor instance variable undefined"
- "instance property undefined constructor uninitialized"
🕗 Version & Regression Information
> npx tsc --version
Version 5.4.3
⏯ Playground Link
💻 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.