Unsafe access to uninitialized variables is allowed in any function

TypeScript Version: 3.3.1

Search Terms: uninitialized variable, unsafe access

Code

function blowup() {
    ohNo.toLocaleLowerCase();
}

let ohNo: string;

blowup();

Expected behavior: This should not compile when using strictNullChecks

Actual behavior: It compiles, probably because the flow analysis abandonned. This convenience is not worth the safety tradeoff, the compiler should enforce that ohNo be typed as string | undefined since the initialization code is not immediately following.

Playground Link:

https://www.typescriptlang.org/play/#src=function blowup() { ohNo.toLocaleLowerCase()%3B } let ohNo%3A string%3B blowup()%3B

Related Issues: Not really. It’s somewhat related to the old bugs around class property initialization not being enforced.

[RyanCavanaugh]

“immediate” as in, direct code statements, not function calls, since functions seem to be the current limitation

What I mean is, the ohYes variable is not immediately initialized in your example, because there’s an intervening possibly-undefined-observing function call between its declaration and its definite initialization. I suspect that will often be the case even for code that is well-formed.

We could add a flag but this could also be syntactically enforced by a lint rule, so I’d generally defer to that route, especially since the definition of “immediate” is really up for debate.

[ziloen]

function someFn() {
  console.log(someVar) // this will cause runtime errors
} 
someFn()
const someVar = 123