microsoft / microsoft/TypeScript
Array.isArray refinement loses the array's type
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Description
I realize there are a number of related issues. However, some where about ReadonlyArray, some didn't see directly related at all even though it was pointed there.
Also, this is a much smaller example, just a single line of code really:
I know people will point to this ir that implementation or design decision detail — honestly guys, you got it REVERSED. The tool should serve the purpose, not the other way around!
That the array loses all it's type information in such a simple example is A BUG.
And it is possible to do much better: See here! I'm only pointing this out because at times the responses sound like "We tried everything but it is just not possible" when the competition shows that it is actually quite possible.
Related(?): #17002
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 by reproducing the single-line example in the linked TypeScript Playground and compare the inferred type before and after Array.isArray refinement. Read the related issue #17002 and the existing discussion for context. Done means the refinement preserves the array's type information in this example and relevant cases.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100