When implementing error handling, ensure comprehensive test coverage by testing multiple error scenarios, edge cases, and boundary conditions. Don't just test the happy path - validate that your code properly handles invalid inputs, out-of-range values, and error propagation. Test cases should verify: - Specific error codes and messages are returned correctly - Custom error properties are preserved during propagation - Edge cases like invalid values (NaN, Infinity, negative numbers, non-integers) - Boundary conditions and out-of-range inputs - Error state transitions (e.g., operations after stream end) Example from the discussions: ```javascript // Test multiple invalid port scenarios for (const value of [NaN, Infinity, -1, -Infinity, 1.1, 9999999]) { // Test each invalid case } // Verify specific error codes res.write('world', (err) => { strictEqual(err.code, 'ERR_STREAM_WRITE_AFTER_END'); }); // Test custom error properties are preserved assert.strictEqual(e.abc, 123); assert.strictEqual(e.stack.includes('at async Object.test'), true); ``` This approach ensures robust error handling that gracefully manages failure scenarios and provides meaningful feedback to users and developers.