Array spreads should be contextually typed

Ryan Cavanaugh (@RyanCavanaugh)

TypeScript Version: 3.8

Search Terms:

array literal type
array spread type inference
implicit array literal type
array destructuring type inference [sic]

Response to Original Issue Post

We have "special" logic for unifying object literals in these cases; the spread syntax interferes with that.

If you post a more representative sample where it's not just spreading for the sake of spreading, we may be able to advise more.

Originally posted by Ryan Cavanaugh (@RyanCavanaugh) in #30761 (comment)

Real Use Case

The common pattern of literal array assignment with conditional inclusion of elements is one use case.

Code

type t = 'a'

// The common pattern
const a:t[] = [
  ...(true?['a']:[]),
]

// Minimal repro of phenomenon
const a:t[] = [
  ...['a']
]

The error messages given above are both Type 'string[]' is not assignable to type '"a"[]'.

Of course if the type of the nested array is asserted or otherwise known, the spread doesn't get in the way of that - it only seems to interrupt the automatic inference of the literal. We get as specific as string, but not as specific as the actual value.

I think that issue is not quite related.

This seems pretty reasonable, though in the future please follow the template.

const a: ("foo" | "bar")[] = [
    ...(Math.random() < 0.5 ? ["foo"] : []),
];

Playground Link

This seems pretty reasonable, though in the future please follow the template.

Yeah sorry - I used the "reference in new issue" button and it pre-filled the above quote instead of the standard template, so I just went with that.

I think that issue is not quite related.

I guess object spread types are inferred in a way analogous to the way in which the array example isn't, so here's another example.

type a = {one: 1}
type b = 'b'[]

const a:a = {...{one:1}} // work
const b:b = [...['b']] // not work

Any updates on this case?

type Case = [string?]

const caseTrue: Case = [...(true ? ["abc" ] : [])];
const caseFalse: Case = [...(false ? ["abc"] : [])];

Let's track at #45600

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

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Array spreads should be contextually typed #42037

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Array spreads should be contextually typed #42037

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DanielRosenwasser commented on Dec 18, 2020

mournie commented on Dec 19, 2020

mournie commented on Dec 19, 2020

rentalhost commented on May 24, 2023

RyanCavanaugh commented on Oct 20, 2025

typescript-bot commented on Oct 23, 2025

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