recipe 14.4. Highlighting Form Field with Missing or Incorrect Data

14.4.1. Problem

You want to highlight missing or incorrectly entered information in a form field.

14.4.2. Solution

For the fields that need to be validated, assign a function to the form field’s onchange event handler that checks whether the field value is valid. If the value is invalid, pop up an alert with information about the error, and highlight the field using a contrasting color: