Uninitialized local check still permits provably erroneous use-before-assign in closure

[kirkwaiblinger]

🔎 Search Terms

uninitialized, local, use before assign, control flow

🕗 Version & Regression Information

• This changed in commit or PR error on variables that are used but never initialized #55887

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.7.0-dev.20240925#code/DYUwLgBAZg9jBcEDOYBOBLAdgcwNwCh8BjGTFZcAMTggF4IAKASkQDcZ0ATOgPggG98ECOiiNYMAHSgcYABYQAPBACMTAUOHQa9AOQTdBYQF98xgvhAAPAA4xUkfqaA

💻 Code

let foo: string;

const setFoo = (): void => {
 if (foo.length < 1) {
 foo = 'foo';
 }
};

export {}

🙁 Actual behavior

No TS Error

🙂 Expected behavior

Variable 'foo' is used before being assigned.(2454)

Additional information about the issue

(this error is correctly reported if we never assign at all, as below)

let foo: string;

const setFoo = (): void => {
 if (foo.length < 1) {
 }
};

export {}

Related, #60064.

Otherwise this can be solved by not considering assignments which occur unconditionally after a read.

[kirkwaiblinger]

Ryan Cavanaugh (@RyanCavanaugh) Sorry to ping, but I see the label has changed to "Awaiting More Feedback" but there's no other discussion. Is there anything you need from me or is discussion waiting on something else?

[RyanCavanaugh]

Neither? For feedback like this we would be wanting to hear from more than one person encountering a particular kind of situation.

[kirkwaiblinger]

Ah, gotcha, ok. I misunderstood what the label meant. Thanks for clarifying!

[horberlan]

class DataProcessor {
 private processedData: string;
 constructor(data: string) {
 if (data.length > 0) {
 this.processedData = this.process(data);
 }
 // Still missing initialization for the empty data case
 }
 logProcessedData() {
 console.log(this.processedData); 
 // We should get error in TypeScript 5.7: Variable 'processedData' is used before it is assigned.
 }
}

[kirkwaiblinger]

poisnada (@horberlan) That does have the error, just on the private processedData line, though, right? I see that error in the playground