User interaction.
HOW EVENTS WORK
Handlers are attached as props, with React managing the underlying listeners.
WHAT TO PASS
A function reference, not a call.
WHAT THAT MISTAKE LOOKS LIKE
The function running immediately on render rather than on interaction.
WHAT CONTROLLED INPUTS ARE
Inputs whose value comes from state, updated on change.
WHAT THAT PROVIDES
A single source of truth, and validation as they type.
WHAT IT COSTS
A render per keystroke, which matters in large forms.
WHAT UNCONTROLLED INPUTS ARE
Inputs managing their own value, read when needed.
WHEN TO USE THEM
Simple forms, and where performance matters.
WHAT TO PREFER FOR SUBSTANTIAL FORMS
A form library, which handles validation, errors and submission state.
WHAT TO ALWAYS DO
Preserve entered values on error Show errors beside the relevant field Disable submission while submitting
WHY THAT LAST ONE
Otherwise people click again and create duplicates.
WHAT TO REMEMBER
Client-side validation is convenience. The server must validate regardless.
