Boundaries and edge cases

What this means

Boundary tests cover empty values, minimum and maximum limits, invalid input, and transitions between branches.

In beginner terms, this topic answers one practical question: "What should I write, and why does React care about it?" Do not try to memorize the syntax first. First understand the idea, then connect the syntax to that idea.

Why it matters

Defects often hide at edges rather than in ordinary middle values.

When you build real React screens, this idea helps you decide where data should live, what the user should see, and what should happen after an interaction. That is why this lesson is part of the main path instead of being an optional detail.

Step by step

1. Notice the UI problem this topic solves. 2. Look at the smallest possible example. 3. Change one value and predict what should appear. 4. Run the example and compare the result with your prediction. 5. Use the practice task before moving on.

Small example

calculatePercent(0, 0); calculatePercent(1, 1);

Common mistake

Do not copy boundaries and edge cases syntax without explaining what problem it solves and checking the result.

Practice task

Change one part of the example, predict the result, run it, and explain the result in your own words.

Remember this

Do not create random edge cases without connecting them to a requirement.