Build a Live Form Validator
React to typing in real time — show errors as the user types, not after they submit. The pattern: input event → validate → toggle an error class.
Step 1 of 3
Listen to input, not submit
The submit event fires once, when the user presses Enter or clicks the submit button. The input event fires on every keystroke, immediately. For live feedback you want input.
Inside the handler, you read event.target.value, validate it, and toggle a class like .error on the input or its wrapper. CSS does the rest: red border, error message visible, etc.
Tip
Live validation should be helpful, not punishing. Wait until the user has typed something before showing errors. A blank field is not yet a mistake — it is unfinished.
Learn more on MDN