Type narrowing with literal index in tuples

[yahiko00]

Search Terms

type inference narrowing literal array tuple

Suggestion

It would be convenient if TypeScript could narrow the type of an item in a tuple, with literal numeric index, especially when the tuple can contain null items.

Use Cases

For fixed length tuples, we often access each item with a literal numeric index.
When an item of such a tuple is null, TypeScript seems not to be able to narrow the item's type after a null check.

Examples

let period: [Date, Date | null] = [new Date(), null];
let dateEndStr = [period[1] ? period[1].toISOString() : null]; // Error: object period can be null

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript / JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. new expression-level syntax)

Related: #10530, which had a fix but the performance hit was too much.