Learn TypeScript

Quiz

This quiz will test what you have learned about conditional types.

What type is Check equivalent to in the code below?

type Animal = {
  name: string;
  legs: number
}
type Check = Animal extends { name: string } ? "yes" : "no";

We have a requirement to create a utility type called NullableProps that gets all the nullable properties from an object type. How could this be defined?

type NullableProps<T> = {

[P in keyof T]: P

}[keyof T];

type NullableProps<T> = {

[P in keyof T]: null extends T[P] ? never : P

}[keyof T];

type NullableProps<T> = {

[P in keyof T]: null extends T[P] ? P : never

}[keyof T];

We have a requirement to create a utility type similar to ReturnType with the improvement of handling asynchronous functions. How could we define this type?

type ReturnTypeAsync<

T extends (...args: any) => any

> = T extends (...args: any) => Promise<infer R>

? R

: T extends (...args: any) => infer R

? R

: any;

type ReturnTypeAsync<

T extends (...args: any) => any

> = T extends (...args: any) => infer R

? R

: any;

type ReturnTypeAsync<

T extends (...args: any) => any

> = T extends (...args: any) => Promise<infer R>

? R

: any;

Creating some standard conditional utility types

Introduction

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