I stumbled upon this type declaration in my codebase which is meant for non-empty arrays:
type NonEmptyArray<T> = T[] & { 0: T }
and it functions as expected:
const okay: NonEmptyArray<number> = [1, 2];
const alsoOkay: NonEmptyArray<number> = [1];
const err: NonEmptyArray<number> = []; // error!
Queries:
1 I'm confused about the significance of 0 in { 0: T }. Can you please explain?
2 What's the difference with the alternative declaration?
type NonEmptyArray<T> = [T, ...T[]];
When using { 0: T }, the value of 0 serves as a key that matches the index of the first element in an array. In JavaScript, arrays are a subtype of objects, so this method works well. The alternative declaration may produce the same outcome, but it is much simpler and easier to comprehend. It's possible that the original code was created for an older version of TypeScript since spreading types in tuples is a relatively new feature.
The code snippet utilizes the generic type T along with an intersection type (indicated by the &). This combination results in a type that includes all the fields of T[] and the type literal { 0: T}. Essentially, it defines an array of type T where the zero index value must also be of type T. In JavaScript, arrays are considered subtypes of objects, with their indices represented as numbers. As a result, any array can be accessed using object-like syntax.
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