Checking for undefined variables is crucial for preventing runtime errors, validating function parameters, and handling optional values safely. As the creator of CoreUI with over 25 years of JavaScript development experience, I’ve implemented undefined checks extensively in component prop validation, API response handling, and defensive programming patterns. From my expertise, the most reliable approach is using strict equality comparison with undefined or the typeof operator for comprehensive undefined detection. This technique ensures robust code that handles missing or uninitialized values gracefully.