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Object unexpectedly undefined #52104
Description
Bug Report
🔎 Search Terms
undefined object, callback, closure, hoisting, arrow function
🕗 Version & Regression Information
- This is the behavior in every version I tried (every version available in the playground, from 3.3.3 to Nightly 5.0.0), and I reviewed the FAQ for entries about
undefined,closure,hoisting
⏯ Playground Link
Playground link with relevant code
💻 Code
const fooMap:Map<string,Array<number>> = new Map()
function someFunction(arg:Array<number>) {
//...
}
const values = [1, 2, 3, 4, 5];
let foo = fooMap.get("a");
if (foo == null) {
foo = [];
}
// The issue is this line, where TS says that `foo` may be undefined
values.forEach((v) => foo.push(v));
// In this line, TS recognizes that `foo` is not undefined
someFunction(foo);🙁 Actual behavior
A variable (in this case, foo) is being marked as possibly undefined when used inside of a closure, although it will never be null at that point.
🙂 Expected behavior
I expect foo to be not-null.
I apologize if the observed behaviour is expected -- but, if it is, can someone explain why?
Activity
MartinJohns commentedon Jan 4, 2023
Duplicate of #9998.
mqualizz commentedon Jan 4, 2023
Is this a duplicate? I've read the bug, and am still reading through all of the comments, but I don't believe this is a situation where control flow analysis is having to contend with a function call.
The problem isn't
someFunction(foo)(which happens after thevalues.forEach()function call). Even if it were to be argued that we can't determine whethervalues.forEach()modifiesfoo, and thusfoocould beundefinedat thesomeFunction()line, that's not what's happening --foois being marked as possiblyundefinedin the callback (and not at thesomeFunction(foo)line).What am I missing?
MartinJohns commentedon Jan 4, 2023
Narrowing is reset in closures. It is a duplicate.
mqualizz commentedon Jan 4, 2023
Ok, thank you.