Validate Single field On LostFocus Any Example?

 Hi,

Coming from a Windows Forms Enviroment and adjusting to the web way of doing things.

Is it possible using some Javascript/Ajax stuff (not that familiar with to be honest) to validate a field as soon as it loses focus?

I have this scenario

When a user types a Credit card number i need to see what kind of card it is and check server Side if they are any "custom Fees defined by the user in set up " attached to the credit card. EG (Visa =£ 2.00 ,Diners =£ 1.00 Fee etc..)

I need to notify the user before hand.Pressing Submit will be too late.

Any examples out there how to validate on LostFocus? Googled it many hits but cannot find what I am looking for

 

Thanks for any suggestions
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