Api
    What is API
    machine readable data (compared to human readable HTML in browsers)

Api
    User goes to http://wa.toad.cz/~username/index.php
        -> Browser HTTP GET /index.php
        -> Server HTTP response HTML
    User fills username field
        -> JavaScript HTTP GET /api/username-avaliable.php
        -> Server response
            data: JSON
            status code
            headers
                Content-Type: application/json
        -> JavaScript reads server statuscode and response
        -> JavaScript parses HTTP response and reads the data
        -> Javascript updated the document