No typechecking error in code that uses an uninitialized variable due to circular reference

[jthemphill]

🔎 Search Terms

uninitialized variable, lexical scoping, circular reference

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about uninitialized variables
• I encountered this error on my company's TypeScript version (5.3.3), and reproduced it on Nightly in the playground.
• I encountered this issue, though it seems to be filled with people asking for fewer errors when their code has circular references😅 T implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer #35546

⏯ Playground Link

https://www.typescriptlang.org/play/?noUncheckedIndexedAccess=true&allowUnreachableCode=true&allowUnusedLabels=true&noUnusedLocals=true&noUnusedParameters=true&target=99&strictBuiltinIteratorReturn=true&useUnknownInCatchVariables=true&exactOptionalPropertyTypes=true&noFallthroughCasesInSwitch=true&noImplicitOverride=true&noPropertyAccessFromIndexSignature=true&suppressImplicitAnyIndexErrors=true&ts=5.7.0-dev.20240822#code/PTAEEkDtQFwCwKagE4IIYBtQGMD2ATBAGlEy1wDNZEBnJCgV0mxgEtdIbTVSaBPZqUj4UCGA2TQADslwBbVnRoBuAFAgIoAFYMaMUHLSFQCAG4JkfeK0gBzUP2ZxZkXLtCIeMXAZusFAF5I8MEIejhodKrqYAAqiKKYoObINOzQlNSKoIzMbByg+LhhEdhwNvZ6DBQUqrks6TliZQDCaGUI+ABqmAxhADxdAHwAFBTNcADykG0dALKKNABcoCMAlKAAvEOgXWsrXaAA3qqgouKSTTBl07OICzQ062oAvtF4nPq2Yi24uBiQMI0ACqaTsdyQm1W+1AACM-hh0NBtsdTjgOOFFL9-lsrq12ohur0wiN1it4f8kVsdiczmcNPFsjA+FIEB1sABrLg6cIUGzEEzmaDwNy2OCgAAGWIREtA2Vc+j8bEwrCCIk8CAAhGizqgLtBpf81GcXmtjecJAaaNiMK9ohoAEpMNhyJAWWTIFZtSAK0jYbBA0AAckNGCDcIQFFwPCVrBVATQ+Wgqm+MBtgMeoIqEOeqiAA

💻 Code

// In the real code, all of these functions are async and return promises;
// I just made everything synchronous here to minimize the test case

// The real version of this function does caching stuff
function fetchCachedValues<V>(fetchOnCacheMiss: () => V): V {
  return fetchOnCacheMiss();
}

const getCoolnessUsingCache = (): boolean => {
  const isCool = fetchCachedValues((): boolean => {
    // This typechecks just fine, even though `isCool` is not initialized here!
    return isCool;
  });
  return isCool;
}

// Runtime error: Cannot access 'isCool' before initialization 
getCoolnessUsingCache();

🙁 Actual behavior

TypeScript reports no errors. Running the code gives a runtime error: Cannot access 'isCool' before initialization

🙂 Expected behavior

I think TypeScript should report an error! Possibly Block-scoped variable 'isCool' used before its declaration.(2448)

Additional information about the issue

No response

[MartinJohns]

Essentially a duplicate of #11498.

[RyanCavanaugh]

If fetchCachedValues defers execution of its callback, this code is fine, but we don't know if it defers or not. If we did error, there'd be no 'correct' way to write correct code, thus there isn't an error.

[jthemphill]

Yeah, after reading #11498 I understand why that would resolve this issue.

I'm mostly surprised, and annoyed, that ECMAScript allows

const x = function() { return x; }

in the first place, and I agree that a defer keyword in TypeScript would let the typechecker differentiate between lambdas that do and don't work with circular references.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.