Custom Jest Matcher: toBeUndefined

Jest provides a robust set of matchers for asserting values, but sometimes you need a custom matcher to fit specific testing scenarios. This challenge asks you to create a custom Jest matcher called toBeUndefined that checks if a value is strictly equal to undefined. This is useful for ensuring that functions or variables return undefined when expected, which is a common pattern in asynchronous operations or optional values.

Problem Description

You need to implement a custom Jest matcher named toBeUndefined. This matcher should extend Jest's expect.matcher functionality. The matcher should take no arguments and should return a result object with pass and message properties. The pass property should be true if the received value is strictly equal to undefined (using ===), and false otherwise. The message property should provide a clear and informative message indicating whether the assertion passed or failed, including the value being tested.

Key Requirements:

• Strict Equality: The matcher must use strict equality (===) to compare the value with undefined.
• Informative Messages: The messages should clearly state whether the value is undefined or not, and include the value itself in the failure message.
• Correctness: The matcher must accurately identify undefined values.
• Jest Compatibility: The matcher must integrate seamlessly with Jest's assertion framework.

Expected Behavior:

• expect(undefined).toBeUndefined() should pass.
• expect(null).toBeUndefined() should fail.
• expect(0).toBeUndefined() should fail.
• expect("").toBeUndefined() should fail.
• expect({}).toBeUndefined() should fail.
• expect([]).toBeUndefined() should fail.

Edge Cases to Consider:

• The value being tested could be any JavaScript data type.
• The matcher should handle cases where the value is not a primitive type (e.g., objects, arrays).

Examples

Example 1:

Input: undefined
Output: { pass: true, message: 'expected undefined to be undefined' }
Explanation: The value is strictly equal to undefined, so the assertion passes.

Example 2:

Input: null
Output: { pass: false, message: 'expected null not to be undefined' }
Explanation: The value is not strictly equal to undefined, so the assertion fails.

Example 3:

Input: 5
Output: { pass: false, message: 'expected 5 not to be undefined' }
Explanation: The value is not strictly equal to undefined, so the assertion fails.

Constraints

• The matcher must be implemented in TypeScript.
• The matcher must extend Jest's expect.matcher functionality.
• The matcher must use strict equality (===) for comparison.
• The matcher should be concise and readable.

Notes

• You'll need to define a type for the matcher's result object.
• Consider using Jest's matcherHint utility to generate helpful hints for users of your matcher.
• Think about how to provide a clear and informative error message when the assertion fails.
• Remember that Jest matchers are functions that receive the actual value as their first argument.