Controlling a generic interface's acceptance of certain data types in typescript: a guide

Question

Controlling a generic interface's acceptance of certain data types in typescript: a guide

Is there a way to restrict a generic interface from accepting specific data types in TypeScript?

I understand that I can define acceptable data types for a generic interface in TypeScript.

interface exampleInterface<T = void | any>;

But what if I want to prevent my interface from accepting one or two specific data types?

Note: I have come across the following question, however it addresses defining what data types ARE allowed while my question focuses on defining what data types are NOT allowed in this generic interface.

Restricting generic types to one of several classes in Typescript

angular typescript web interface

Answer 1

Answer №1

To achieve this, you can create a type in the following way:

type NotAcceptable<T> = T extends string | undefined ? never : T

With this definition, values of type string or undefined will not be accepted.

You can now use it as shown below:

let t1: NotAcceptable<string> = "abc"  // Error

function test<T>(t: NotAcceptable<T>){}

test("string")  // Error
test(undefined) // Error
test(123)       // Ok

The only drawback is that this is a type and not an interface, as specified in your question. However, you can still utilize types like interfaces and even add additional properties:

type NotAcceptable<T> = T extends string | undefined ? never : {
  abc: T
}

Playground

Answer 2

To achieve this, you can create a type in the following way:

type NotAcceptable<T> = T extends string | undefined ? never : T

With this definition, values of type string or undefined will not be accepted.

You can now use it as shown below:

let t1: NotAcceptable<string> = "abc"  // Error

function test<T>(t: NotAcceptable<T>){}

test("string")  // Error
test(undefined) // Error
test(123)       // Ok

The only drawback is that this is a type and not an interface, as specified in your question. However, you can still utilize types like interfaces and even add additional properties:

type NotAcceptable<T> = T extends string | undefined ? never : {
  abc: T
}

Playground

Controlling a generic interface's acceptance of certain data types in typescript: a guide

Answer №1

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