Object unexpectedly undefined

[mqualizz]

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?

[MartinJohns]

Duplicate of #9998.

[mqualizz]

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 the values.forEach() function call). Even if it were to be argued that we can't determine whether values.forEach() modifies foo, and thus foo could be undefined at the someFunction() line, that's not what's happening -- foo is being marked as possibly undefined in the callback (and not at the someFunction(foo) line).

What am I missing?

[MartinJohns]

Narrowing is reset in closures. It is a duplicate.

[mqualizz]

Narrowing is reset in closures. It is a duplicate.

Ok, thank you.