Type guards ignored in closure

[hgl]

TypeScript Version:
v4.0.0-dev.20200523

Search Terms:
type guard closure
Code

let data: {[k: string]: string} = {}
function deleteOnClick() {
 let key = findKey()
 if (!key) {
 return;
 }
 let key2 = key
 window.addEventListener("click", () => {
 delete data[key2] // this is ok
 delete data[key] // type error, key can be null
 })
}

function findKey(): string | null {
 return null
}

Expected behavior:
delete data[key] contains no error
Actual behavior:
delete data[key] contains type error

Playground Link:
https://www.typescriptlang.org/play/?#code/DYUwLgBAJghmMC4IG8DaBrJBnMAnAlgHYDmAutnkcQL4QC8K1AUAGYCuhAxmPgPaHQQoMCADyhAMLB8ndAAoAlCiYRVEYRHQgAnvQgsiUANI7FKtfhYQ5AQi3alyc2rW5wbXIQDcziMxca9gBMeva+AO6GvOEAdDBQUACiAG4ghGAAMvg4aSC4cgBEnNKyBQA01kp0AHzKLi5QQuAg0HAwGDpBpBAA9D0QYAAW2RAjvOi+DU0irfAd2t19A9oADi15uLy4FfYQnDACAEYthGzAwL7UCkzMrBzcfAIGhMamChQEJBAAPhCn53VXO5PH8zhdqEA
Related Issues:

[hgl]

Asked on SO, doesn't get any response. Looks like a bug to me.

[MartinJohns]

Duplicate of #9998.

[hgl]

Thanks

[hgl]

On second thought this doesn’t seem to be a duplicate. key is locally defined, there is no way the addEventListener call can reset it back to null.

Not sure it’s flow control related.

[ahejlsberg]

This is a design limitation. When a local variable is declared with let or var the control flow analyzer assumes that there could be subsequent assignments to the variable following the creation of a closure that references the variable, and therefore that narrowed types may no longer be correct (it's just that there aren't any such assignments in your example). Your example works if you use const in the declaration of key because it is then known that there won't be any further assignments and that it is safe to carry the narrowed types into the closure.

We could theoretically do more work in the control flow analyzer to determine that there are no assignments to a particular variable following the creation of a function closure that references the variable, but it's not trivial.