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It is not allowed to push a number to the array foo in {foo: []}

See original GitHub issue

TypeScript Version: Playground

Search Terms: never[]


const foo = { foo: [] };;

const bar = [];

Expected behavior: TypeScript should infer foo to be of type { foo: any[] } and therefore; should be allowed like bar.push(1); is allowed

Actual behavior: Line 2: Argument of type ‘1’ is not assignable to parameter of type ‘never’.

Playground Link: foo %3D { foo%3A [] }%3B const bar %3D []%3B bar.push(1)%3B

Related Issues:

Issue Analytics

  • State:open
  • Created 5 years ago
  • Comments:5 (3 by maintainers)

github_iconTop GitHub Comments

AnyhowStepcommented, Jan 14, 2019

I tend not to use const x = [];. When I do need to initialize a variable to an empty array, I tend to be explicit with the data type.

So, I’d use, const x : /*insert-data-type-here*/[] = []

And never[] is assignable to /*insert-data-type-here*/[] because never is assignable to all types.

Seems like @fatcerberus has a link that shows what you are seeing is intended behaviour, though

fatcerberuscommented, Jan 14, 2019

For bar, TS “evolves” the type with each .push(). So it starts out as any[] but then evolves to number[]. Push a string afterwards and it may then evolve to (number | string)[], etc. I don’t believe that works with [] in object literals, though–after the initial definition (which is inferred as never[] for some reason), the type cannot change further.


A variable declared with no type annotation and an initial value of [] is considered an implicit any[] variable. However, each subsequent x.push(value), x.unshift(value) or x[n] = value operation evolves the type of the variable in accordance with what elements are added to it.

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