I'm unfamiliar with Typescript and struggling to grasp the significance of this syntax. Could someone provide some clarification?
type Type1<K> = K extends string ? { [P in K]: string } : never;
If type K is a subclass of string, does that mean Type1 will result in an array of strings? For example:
{"x": ["a", "b", "c"],
"z": ["d", "e", "f"]
//and so on
}
Or
{"x": "a",
"z": "b"
//and so on
}
First, let's examine { [P in K]: string }, which is a mapped type. If K consists of string literal types (e.g. 'a' | 'b'), the resulting type will be an object with those names as keys and values of type string (such as { a: string, b: string }). Essentially, this is the same as the built-in Record type.
The syntax K extends string ? ... : never represents a distributive conditional type, as explained in TypeScript documentation. This means that if K is a union type, each member of the union will undergo the mapped type transformation. For example:
type ex = Type1<'a' | 'b'> =>
('a' extends string ? { [P in 'a']: string } : never) | ('b' extends string ? { [P in 'b']: string } : never) =>
({ [P in 'a']: string }) | ({ [P in 'b']: string }) =>
{ a: string } | { b: string }
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