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Tuple iteration and merging #26058

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@dead-claudia

Description

@dead-claudia

Edit: Remove indexing part
Edit 2: Replace tuple mapping syntax with normal mapped types

Search Terms

Concatentate and merge tuples' entries.

Suggestion

  1. I would like to be able to extract and spread in positions other than the last part of a tuple.
  2. I would like to merge a tuple into an intersection or concatenation of its members.

Use Cases

  • Converting a tuple to its intersection, such as with Object.assign's assignee.
  • Typing Array.prototype.concat correctly for tuples.

Syntax

The syntax comes in a few new forms:

  • [...A, B] appends B to the tuple A. Similarly, [...infer A, any] extends B ? A : [] drops the last item of the tuple and [...any[], infer A] extends B ? A : never extracts the first.
  • {... ...T} for an n-ary merge and [... ...T] an n-ary concatenation.

Examples

Here's the types for each method I mentioned above.

interface Array<T> {
    concat<U extends any[]>(...others: U): [...this, ... ...{[I in keyof U]: (
        U[I] extends any[] ? N : U[I] extends ArrayLike<infer R> ? R[] : [U[I]]
    )}];
}

interface ObjectConstructor {
    assign(target: {... ...typeof sources}, ...sources: object[]): typeof target;
}

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)

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