Happy Paths and Edge Cases

When you build a feature, you probably imagine users doing exactly what you expect. They enter reasonable data, click buttons once, and have perfect internet connections. This ideal scenario is called the "happy path." Real users, however, are wonderfully unpredictable.

What Is the Happy Path?

The happy path is the expected flow through your application when everything goes right:

User enters a valid todo: "Buy groceries"
Frontend sends request to backend
Backend saves to database
Backend returns success
Frontend displays the new todo

If you only test the happy path, your application will break the moment someone does something unexpected.

What Are Edge Cases?

Edge cases are unusual, extreme, or unexpected inputs and situations. They live at the "edges" of what your code handles:

Empty input (user submits nothing)
Very long input (10,000 characters)
Special characters: <script>alert('xss')</script>
Unicode: emojis 🎉, right-to-left text
Rapid actions: clicking submit 50 times in a second
Network issues: slow or missing connection
Concurrent actions: two users editing the same item

Finding Edge Cases

You won't think of every edge case on your own — that's normal. AI excels at generating comprehensive lists:

"What edge cases should I handle for a todo creation feature?"

AI will suggest cases you hadn't considered. Not all will be relevant to your application, but many will reveal potential bugs.

Thinking Defensively

Defensive coding means assuming inputs will be weird. Instead of trusting that users provide valid data, verify everything:

def create_todo(text):
    # Don't trust the input
    if not text:
        raise ValueError("Todo text cannot be empty")
    if len(text) > 500:
        raise ValueError("Todo text too long")
    # Now safe to proceed

This mindset catches problems before they cause confusing errors deeper in your code.

Graceful Degradation

When edge cases occur, your application should fail gracefully. Instead of crashing or showing a blank screen, provide helpful feedback:

"Please enter some text for your todo"
"Todo text is too long (maximum 500 characters)"
"Unable to save — please check your connection"

Users can recover from clear error messages. They can't recover from a frozen screen.

Prioritizing Edge Cases

You can't handle every possible edge case immediately. Prioritize:

Security issues (like script injection) — handle immediately
Data corruption (like saving invalid data) — handle before launch
User experience (like slow networks) — improve over time

Start with the happy path, then systematically address edge cases based on their impact.

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