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HTML5 Data and Services Cookbook by Mite Mitreski, Gorgi Kosev

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Using asynchronous server-side validation

Many validation checks can only be performed at the server side. The following are the examples:

  • When validating a user registration form, we need to check if the entered username is available
  • When the user enters a postal address, we might need to ask an external service to verify if the address is correct

The problem with server-side validation checks is that they need to be asynchronous. As a result, they cannot be written in JavaScript as functions that return validation results.

To solve this problem, in this recipe we're going to make a validator that uses the continuation-passing style. The example has a username input field that is validated against the server. The server checks if the username is ...

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