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

TypeScript Version: Playground

Search Terms: never[]

Code

const foo = { foo: [] };
foo.foo.push(1);

const bar = [];
bar.push(1);

Expected behavior: TypeScript should infer foo to be of type { foo: any[] } and therefore foo.foo.push(1); 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: https://www.typescriptlang.org/play/index.html#src=const foo %3D { foo%3A [] }%3B foo.foo.push(1)%3B const bar %3D []%3B bar.push(1)%3B

Related Issues:

[AnyhowStep]

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.

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

[fatcerberus]

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.

Source: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html

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.