microsoft / microsoft/TypeScript
Array.length type guard for array spreading
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Description
TypeScript Version: 3.3.0-dev.20181204
Search Terms: array length type guard spread
Code
const timeString = '09:00';
const date = new Date();
const timeParts = timeString.split(':').map((part) => parseInt(part, 10));
if (timeParts.length > 0) {
date.setHours(...timeParts);
}
Expected behavior: No errors
Actual behavior: TS2556: Expected 1-4 arguments, but got 0 or more.
Related Issues: None
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 TypeScript 3.3.0-dev.20181204 example from the linked Playground, focusing on array spreading after an Array.length check. Trace the compiler's array-spread argument checking and length narrowing, then add a regression test and confirm the guarded spread no longer reports TS2556.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100