Object is possibly undefined inside a closure

[iby]

TypeScript Version: 3.7.0-dev.20190907

Search Terms: possibly undefined is:open

Code

function foo(value?: string) {
    if (value === undefined) { value = "hello"; }
    console.log(value.toUpperCase()); // OK
    setTimeout(() => value.toUpperCase(), 1); // TS2532
}

Expected behavior: Successful compilation.

Actual behavior: Compilation failing with error TS2532: Object is possibly 'undefined'..

Playground Link: www.typescriptlang.org/play/#code/…

Related Issues: #31060

[jack-williams]

Duplicate of #9998

[iby]

@jack-williams What are the odds finding that as a duplicate! 😄 Thanks!