Improve `Array.from(tuple)` and `[...tuple]`

[KSXGitHub]

Suggestion

Array.from(tuple) and [...tuple] should preserve individual types that made up tuple.

Proposal

Array.from (Simple)

Add this overload to Array.from:

interface ArrayConstructor {
  from<T extends any[]> (array: T): T
}

demo 1

Caveats:

• The above definition preserves everything including unrelated properties that do not belong to Array.prototype whilst actual Array.from discards them. (for instance, if input has foo: 'bar', output array will also has foo: 'bar').

Array.from (Complete)

Fix above caveats.

interface ArrayConstructor {
  from<T extends any[]> (array: T): CloneArray<T>
}

type CloneArray<T extends any[]> = {
  [i in number & keyof T]: T[i]
} & {
  length: T['length']
} & any[]

demo 2

Spread operator

declare const tuple: [0, 1, 2] & { foo: 'bar' }

// $ExpectType [string, string, 0, 1, 2, string, string]
const clone = ['a', 'b', ...tuple, 'c', 'd']

Note that typeof clone does not contain { foo: 'bar' }.

Use Cases

• Clone a tuple without losing type information.

Examples

Clone a tuple

const a: [0, 1, 2] = [0, 1, 2]

// $ExpectType [0, 1, 2]
const b = Array.from(a)

Clone a generic tuple

function clone<T extends any[]> (a: T): T {
  return Array.from(a)
}

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)

[aleclarson]

Bump. The current behavior doesn't make any sense to me.

Missing search keyword: spread, spreading

[jcalz]

I'm not seeing how this proposal addresses [...tuple]. I only see Array.from(tuple) discussed. What am I missing?

[aleclarson]

@jcalz I believe the proposed behavior for spreading tuples is:

[...tuple] should return typeof tuple instead of tuple[number][].

@KSXGitHub Can you add this to the OP please?

[KSXGitHub]

I'm not seeing how this proposal addresses [...tuple]. I only see Array.from(tuple) discussed. What am I missing?

@KSXGitHub Can you add this to the OP please?

I will!

@jcalz I believe the proposed behavior for spreading tuples is:

[...tuple] should return typeof tuple instead of tuple[number][].

No, not typeof tuple. You did not consider the possibility that tuple may have non-array properties, for example:

declare const tuple: [0, 1, 2] & { foo: 'bar' } // typeof tuple → [0, 1, 2] & { foo: 'bar' }
const clone = [...tuple] // should be [0, 1, 2] and without { foo: 'bar' }

[KSXGitHub]

By adding "Spread operator" section (as suggested by #27859 (comment) and #27859 (comment)), this issue is no longer within the boundary of lib.d.ts but requires changes from TypeScript engine itself. Should I move the section to a new issue or should I keep it here?

[jcalz]

It looks like #36861 handles tuple spread... but only on pure tuple types like [0, 1, 2] and not those intersected with objects like [0, 1, 2] & { foo: 'bar' } in the example. Cross-referencing anyway:

declare const a: [0, 1, 2];
const b = [-2, -1, ...a, 3, 4, 5] as const;
// const b: readonly [-2, -1, 0, 1, 2, 3, 4, 5]

declare const c: [0, 1, 2] & { foo: "bar" };
const d = [-2, -1, ...c, 3, 4, 5] as const;
// const d: readonly (0 | 3 | 1 | 2 | -2 | -1 | 4 | 5)[]

Playground link

[jakebailey]

Looking at old issues, interestingly as of TS 4.2, the above example is now:

declare const a: [0, 1, 2];
const b = [-2, -1, ...a, 3, 4, 5] as const;
//    ^?
// const b: readonly [-2, -1, 0, 1, 2, 3, 4, 5]

declare const c: [0, 1, 2] & { foo: "bar" };
const d = [-2, -1, ...c, 3, 4, 5] as const;
//    ^?
// const d: readonly [-2, -1, ...(0 | 1 | 2), 3, 4, 5]

Playground Link

So, it gets the two ends correct.