How to validate a textbox inside the update panel javascript?

Pure Javascript validation is almost certainly wrong in ASP.NET, it only checks on the client, not the server, and a this is insecure.

ASP.NET has validation controls built in:
MSDN Validation Controls[^]
All of these generate the JavaScript and validate on the server too (except the CustomValidator: You write the server and client scripts for yourself). The validators are:

• RequiredFieldValidator- The validated field must not be blank
• CompareValidator - Compares against a value, or value of another control
• RangeValidator - Makes sure the value is in a pre-defined range
• RegularExpressionValidator - Makes sure the value matches a regex
• CustomValidator - Complex Validation you define yourself.
• ValidationSummary - Allows you to summarise validation failures