interface extends Array need initial index key

[aaabinbin]

I want to achieve a fixed size array

type FixedLengthArray<T extends any, L extends number> = Array<T> & {
  0: T,
  length: L;
}

type ArrayValue = 'a' | 'b'

type Foo = FixedLengthArray<ArrayValue, 2>;
const foo1: Foo = ['a', 'b'];

But when I remove 0: T, it throws a error, and Array has defined [n: number]: T, what does '0' means.

Type '("a" | "b")[]' is not assignable to type 'FixedLengthArray<ArrayValue, 2>'.
  Type '("a" | "b")[]' is not assignable to type '{ length: 2; }'.
    Types of property 'length' are incompatible.
      Type 'number' is not assignable to type '2'.

[dragomirtitian]

Related to #26223

Why not use a tuple type ?

type Foo = [ArrayValue, ArrayValue]
const foo1: Foo = ['a', 'b'];

Playground Link

My guess as to the error is that typescript uses the presence of a numeric 0 property as a marker to for inferring tuples from array literals. When inferring a tuple the length property will be a numeric literal type (2 in this case). If the property is missing you end up with an array type from the array literal and thus the length property will just be the regular number instead of 2

[aaabinbin]

@dragomirtitian thx~ for your answer, I have found the related issue here