microsoft / microsoft/TypeScript
`String.prototype.split()` with a regex containing a capturing group can return an array of `string | undefined`.
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Description
⚙ Compilation target
any
⚙ Library
esnext, dom
Missing / Incorrect Definition
Using split with a negative lookahead and a capturing group can return undefined. See the attached screenshot. When iterating over the array, we lose type safety and encounter runtime errors.
Sample Code
'a,b,c'.split(/,(?!(a))/) // Types as string[]
Documentation Link
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 at the String.prototype.split entry point in the esnext and dom library definitions and compare its current return type with the sample 'a,b,c'.split(/,(?!(a))/). Check how capturing groups that do not participate are represented at runtime, then add coverage showing that the resulting array type accounts for undefined.
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
- 45/100