Jestype - TypeScript Type Tester for Jest

Jestype is a TypeScript utility designed to facilitate type testing within Jest, providing developers with tools to assert and validate type behaviors in TypeScript projects. At its core, expectType serves as the type-level counterpart to Jest's expect function, enabling type assertions in a manner analogous to value-based testing.

Getting Started

Install Jestype in your development environment with npm:

npm install jestype --save-dev

Then, import and use expectType in your Jest tests to validate types.

expectType

expectType is the primary function of Jestype and acts as the type-level equivalent of Jest's expect.

It allows type assertions for a specific type T, providing a fluent API for asserting type properties and relationships.

Usage

import { expectType } from 'jestype';

// Assert that a type is exactly another type
expectType<string>().toBe<string>();

// Assert that a type extends another type
expectType<string>().toExtend<string | number>();

// Negation example
expectType<string>().not.toBe<number>();

Methods

• .toBe<S>(): Asserts that the tested type T is exactly the same as type S.

• .toExtend<S>(): Asserts that the tested type T extends type S.

• .toBeOfUnion<S>(): Asserts that the tested type T is part of the union S

• .toHave<S>(): Asserts that the tested type T has the same properties as type S.

• .toBePartOf<S>(): Asserts that the tested type T is a subset of type S.

• .toHaveRequired<K extends string[]>(): Asserts that the tested type T has all the required keys specified in K.

• .toHaveOptional<K extends string[]>(): Asserts that the tested type T has all the optional keys specified in K.

• .not: Inverts the subsequent type assertion, allowing for negated checks.

Examples

.toBe<S>

Asserts that the tested type T is exactly the same as type S.

// Should pass: string is exactly a string
expectType<string>().toBe<string>();

// Should fail: string is not a number
expectType<string>().toBe<number>();

---

.toExtend<S>

Asserts that the tested type T extends type S.

/* Using Interfaces*/
interface Base {
    baseProp: string;
}

interface Extended extends Base {
    extendedProp: number;
}

// Should pass: Extended extends Base
expectType<Extended>().toExtend<Base>();

// Should fail: Base does not extend Extended
expectType<Base>().toExtend<Extended>();


/* Using Unions */

// Should pass: string is assignable to string | number 
expectType<string>().toExtend<string | number>();

// Should fail: string | number is not assignable to string 
expectType<string | number>().toExtend<string>();

---

.toBeOfUnion<S>

Asserts that the tested type T is part of the union S

Exactly the same as toExtend, but lends to a more intuitive sounding type assertion for unions

// Should pass: `string` is part of union `string | number`
expectType<string>().toBeOfUnion<string | number>();

// Should fail: `string` is not in union `string | number`
expectType<string | number>().toBeOfUnion<string>();

---

.toHave<S>

Asserts that the tested type T has the same properties as type S.

interface Person {
    name: string;
    age: number;
}

// Should pass: Person has name and age properties
expectType<Person>().toHave<{ name: string; age: number; }>();

// Should fail: Person does not have a salary property
expectType<Person>().toHave<{ salary: number; }>();

---

.toBePartOf<S>

Asserts that the tested type T is a subset of type S.

interface Vehicle {
    make: string;
    model: string;
    year: number;
}

interface Car {
    make: string;
    model: string;
}

// Should pass: Car is part of Vehicle
expectType<Car>().toBePartOf<Vehicle>();

// Should fail: Vehicle is not a subset of Car
expectType<Vehicle>().toBePartOf<Car>();

---

.toHaveRequired

Asserts that the tested type T has all the required keys specified in K.

interface User {
    id: number;
    name: string;
    email?: string;
}

// Should pass: User has required keys id and name
expectType<User>().toHaveRequired<['id', 'name']>();

// Should fail: User does not have a required key 'password'
expectType<User>().toHaveRequired<['password']>();

---

.toHaveOptional<S>

Asserts that the tested type T has all the optional keys specified in K.

interface Product {
    id: number;
    name: string;
    description?: string;
    price?: number;
}

// Should pass: Product has optional key 'description'
expectType<Product>().toHaveOptional<['description']>();

// Should fail: Product does not have an optional key 'weight'
expectType<Product>().toHaveOptional<['weight']>();

---

.not

Inverts the subsequent type assertion, allowing for negated checks.

// Should pass: string is not a number
expectType<string>().not.toBe<number>();

// Should fail: string is not a number
expectType<string>().<number>();

Type Utilities

Jestype also includes Utility Types that expectType is built on:

Logical Types

• And<T, U>: Logical AND of T and U.

• Or<T, U>: Logical OR of T and U.

• Not<T>: Logical NOT of T.

• Nand<T, U>: Logical NAND of T and U.

• Nor<T, U>: Logical NOR of T and U.

• Xor<T, U>: Logical XOR of T and U.

• Eq<T, U>: Checks if T and U are equivalent.

• All<Items>: True if all Items are true.

• Any<Items>: True if any of Items are true.

• None<Items>: True if none of Items are true.

• Switch<Condition, Inverted>: Evaluates Condition and inverts the result if Inverted is true.

• If<Condition, Then, Else>: Returns Then if Condition is true, otherwise Else.

ExtendsTypes

• Extends<T, S>: Determines if type T extends type S.

• BiExtends<T, S>: Determines if types T and S are mutually extendable (bi-directional extends).

Key Types

• RequiredKeys<T>: Extracts the required keys of type T.

• OptionalKeys<T>: Extracts the optional keys of type T.

• AlwaysKeys<T>: Extracts keys of T that are always present.

Primitive Types

• IsAny<T>: Determines if type T is any.

• IsNever<T>: Determines if type T is never.

• IsUnknown<T>: Determines if type T is unknown.

• IsEmpty<T>: Determines if type T is an empty object ({}).

• IsPrimitive<T>: Determines if type T is a primitive type.

Branding

• Brand<T>: Creates a branded representation of type T, useful for complex type analysis.