Jest Custom Matcher: toBeInstanceOf

You're tasked with extending Jest's assertion capabilities by creating a custom matcher, toBeInstanceOf. This matcher will allow developers to assert that a received value is an instance of a specific class or constructor function, mirroring Jest's built-in expect.any() and expect.objectContaining() but with a focus on class instantiation.

Problem Description

The goal is to implement a custom Jest matcher named toBeInstanceOf. This matcher should take a constructor function or class as an argument and return true if the received value is an instance of that constructor/class, and false otherwise.

Key Requirements:

• The matcher should be named toBeInstanceOf.
• It must accept one argument: the constructor function or class to check against.
• It should return true if received instanceof constructor evaluates to true.
• It should return false otherwise.
• The matcher should provide informative error messages when an assertion fails.

Expected Behavior:

When used in a Jest test:

expect(new MyClass()).toBeInstanceOf(MyClass); // Should pass
expect({}).toBeInstanceOf(MyClass); // Should fail
expect(null).toBeInstanceOf(MyClass); // Should fail
expect(undefined).toBeInstanceOf(MyClass); // Should fail

Examples

Example 1:

// In your test file:
class Dog {}
const myDog = new Dog();

expect(myDog).toBeInstanceOf(Dog);

Output: The assertion passes.

Explanation: myDog is indeed an instance of the Dog class.

Example 2:

// In your test file:
class Cat {}
const myDog = new Dog(); // Assuming Dog class from Example 1

expect(myDog).toBeInstanceOf(Cat);

Output: The assertion fails. The error message should indicate that myDog is not an instance of Cat.

Explanation: myDog is an instance of Dog, not Cat.

Example 3:

// In your test file:
class Animal {}
const notAnInstance = {}; // A plain object

expect(notAnInstance).toBeInstanceOf(Animal);

Output: The assertion fails. The error message should indicate that notAnInstance is not an instance of Animal.

Explanation: A plain JavaScript object is not an instance of the Animal class.

Constraints

• The solution must be implemented in TypeScript.
• The custom matcher should be compatible with Jest's testing framework.
• The constructor function passed to toBeInstanceOf will always be a valid function or class.
• The received value can be any JavaScript type.

Notes

• You will need to register your custom matcher with Jest. Refer to the Jest documentation on Extending Jest for guidance on how to do this.
• Consider how to provide clear and helpful error messages for both the passing and failing cases. Jest's custom matcher API provides methods like message and negatedMessage for this purpose.
• Think about the core JavaScript operator that can be used to check for instance relationships.