Update

var Foo = function(){
    this.stage = 0;
};

Foo.prototype.bar(){
    $.ajax({
        url:          url,
        data:         {'stages': this.stage},
        type:         'GET',
        dataType:     'html',
        contentType:  'application/json',
        success:      function(data) {
            //use data
        },
        error:       function(xhr, testStatus, error) {
            console.log("Error occured: "+error+" "+xhr+" "+testStatus);
        }
    });
    this.stage++;            //doesn't work
    console.log(this.stage); //this works
}