Solution #6 - redefine the handler

Another idea: instead of changing anything in the button, let's have the event handler change itself. The trick is in the second line; by assigning a new value to the billTheUser variable, we are actually dynamically changing what the function does! The first time you call the function, it will do its thing... but it will also change itself out of existence, by giving its name to a new function:

function billTheUser(some, sales, data) {
    billTheUser = function() {};
    window.alert("Billing the user...");
    // actually bill the user
}

There's a special trick in the solution. Functions are global, so the line billTheUser=... actually changes the function's inner workings; from that point on, billTheUser will be the new (null) function. This solution is still hard to test. Even worse, how would you restore the functionality of billTheUser, setting it back to its original objective?