如何声明为 AnyRecord 或 object 数组的 Typescript 类型？

Question

In short, I want to do this:

interface Rec {
  key: string;
}

type RecordOrList = Record<string, any> | Rec[];

So, RecordOrList is either some object, or a list of specific objects.

However, typescript here seems to treat arrays of Rec -s as Any Records. For example, this is valid code, but I was expecting it to be a type error ("int instead of string").

const x: RecordOrList = [{
  key: 123,
}];

https://www.typescriptlang.org/play?ts=4.5.4#code/JYOwLgpgTgZghgYwgAgEoQcg3gWAFDLIDWEAngFzIDOYUoA5gNz4C+++YpADiuggPZQAJgHkoAGWA1kAXjQZBQgDw06IegBpkcEKQB8yAD7yEAbQC6zPPgEhpAD0p9FYydLmncBYmUoBGACYAZg1WSyA

How can I make the code above type checked?

Answer 1

The issue you're having here is that arrays themselves are - surprisingly - a valid Record<string, any> (specifically, it's a Record<"length", number> ).

// Valid. WTF?
const x : Record<string, any> = [];
const a : keyof [] = "length"; // Valid
const b : keyof [] = "bogus";  // Invalid
Object.getOwnPropertyNames(x); // => ["length"]

Instead of any , you could use unknown here:

interface Rec {
  key: string;
}

type RecordOrList = Record<string, unknown> | Rec[];

// Type error as expected
const x: RecordOrList = [{
  key: 123,
}];

// Works just fine
const y: RecordOrList = {key: 123}
const z: RecordOrList = [{
  key: "123",
}];

Or just narrow your Record value type to a list of scalar types.