Typescript rest parameter with two possible array types

Question

I would like to define a function with a rest parameter of either strings or objects, signature like:

public static fn(...messages: string[] | object[]): void;

But unfortunately this results in a TS2370 compile error

error TS2370: A rest parameter must be of an array type.

I know having a single type of array like string[] or object[] works fine, but then overloading such function comes with even greater price.

Is there anyway I could make the mentioned function working with the desired signature?

Shane · Accepted Answer · 2017-12-19 21:49:03Z

12

Yes, you can have multiple types for your rest parameter. However, you must declare it as a single array type:

declare function fn(...messages: (string | object)[]): void;

// or with custom type:
// type MessageType = string | object;
// declare function fn(...messages: MessageType[]): void;

fn('aaa', { });

Otherwise you are saying it can accept an array of strings or an array of objects, however it only accepts 1 typed array (combining string and object makes it 1 type).

answered Dec 19, 2017 at 21:49

Shane

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