Quick AJAX / JSON security question

In a form I have I want to be able to check whether the i开发者_开发知识库ntended email address or username is unique as part of the validation process

I have created a JSON file with PHP from database data to work with JavaScript validation but the problem is that anybody can see the JSON file

Is there a way around this and still be able to use this method?

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In short, no.

Why don't you just create an ajax function that takes in the email address and returns a boolean indicating whether it's unique by querying the database? JQuery has great AJAX capabilities: http://api.jquery.com/category/ajax/