Writing Unit Tests with Jest

What Jest feature enables custom assertion creation?

expect.extend({
  toBeWithinRange(received, floor, ceiling) {
    const pass = received >= floor && received <= ceiling;
    return {
      pass,
      message: () => `expected ${received} to be within range ${floor} - ${ceiling}`
    };
  }
});

Matcher extension
This demonstrates custom matcher creation: 1) Extends Jest's assertion capabilities, 2) Enables domain-specific assertions, 3) Improves test readability, 4) Reusable across tests, 5) Provides custom error messages, 6) Maintains consistent testing patterns, 7) Useful for complex validations, 8) Enhances testing expressiveness.
Assertion plugins
Custom validation
Test helpers

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