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Type guard cannot remove undefined from Partial<T>[keyof T] | undefined #45257
Description
Bug Report
When writing a function to merge object A with non-undefined values from object B into C where A is of type T and B is of type Partial<T>, I came across a situation where the type guard seems to be failing.
🔎 Search Terms
- type guard
- Partial
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about common-bugs-that-arent-bugs
⏯ Playground Link
Playground link with relevant code
💻 Code
function merge<T>(a: T, b: Partial<T>, keys: (keyof T)[]): T {
const v: T = { ...a };
for (const key of keys) {
const value = b[key];
if (value !== undefined) {
v[key] = value;
}
}
return v;
}🙁 Actual behavior
There is an error at v[key] = value;:
Type 'T[keyof T] | undefined' is not assignable to type 'T[keyof T]'.
Type 'undefined' is not assignable to type 'T[keyof T]'.
Clearly value is NOT undefined, yet the compiler complains that it is.
🙂 Expected behavior
I would expect that the code compiled.
Activity
puchm commentedon Jul 31, 2021
I am having a similar issue. Here is another example:
The upper one gets the following error at the
obj[key]line:Type 'number' is not assignable to type 'T[Extract<keyof T, string>]'The lower one does not have this issue. This indicates to me that the issue has something to do with the generics.
ahejlsberg commentedon Aug 1, 2021
The issue here is that we only perform control flow analysis for element accesses with literal property names (e.g.
obj['foo']). So, no narrowing is performed forobj[key]because it has a computed property name. We could potentially consider narrowing such expressions when the index expression is a reference to aconstvariable. I'll mark this issue as a suggestion for that (though I wouldn't be surprised if we already have this suggestion).ahejlsberg commentedon Aug 1, 2021
Actually, sorry, I misread the example. The issue is that the guard
value !== undefineddoesn't narrow typePartial<T>[keyof T]toT[keyof T]. I'm not sure how feasible it is to do that narrowing, but it is something we could consider.ilyakonrad commentedon Mar 31, 2025
I have a similar issue.