microsoft / microsoft/TypeScript
Destructuring assignment with computed property name does not help narrow types as intended.
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Description
### 🔎 Search Terms
"destructuring assignment", "computed property name", "type narrowing"
### 🕗 Version & Regression Information
Happen to all existing version in typescript playground.
### ⏯ Playground Link
https://tsplay.dev/w1eElw
### 💻 Code
```ts
declare let obj: Record
declare let cond: boolean;
declare let key: string;
let result: string | null = null;
if (cond) {
({ [key]: result } = obj);
result;
// ^?
}
if (cond) {
({ 'key': result } = obj);
result;
// ^?
}
```
### 🙁 Actual behavior
Type narrowing did not happen to the assignee variable `result` after the assignment `({ [key]: result } = obj);`.
### 🙂 Expected behavior
Expecting the assignment `({ [key]: result } = obj);` will narrow the assignee variable `result` to non-null.
### Additional information about the issue
_No response_
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked TypeScript Playground reproduction and compare the inferred type of result after the computed-property destructuring assignment and the literal-property assignment. Trace the type-narrowing behavior involved and verify that the completed change preserves the expected non-null narrowing in both examples.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100