Using an external file on a button in html

You can put the code in a function inside the javascript. Include the file in script tags and on click of the button, call that function. That will trigger the JS code only on click of the button.

Put this in the HTML,

<script src='button javascript.js'></script>
//Intermediate HTML code
<button onclick = "myButtonFun()">
What do you drive? </button> 

And in the Javascript you can put in your code in the function myButtonFun()

function myButtonFun()
{
  var car = prompt('What car?');

  switch(car) {
  case 'ferrari': document.write("you are probs a baller then");
                  break;
  case "porsche": document.write("do you ball hard?");
                  break;
  case "dodge":   document.write("american cannot corner");
                  break;
  default:        document.write("your car better be a subaru");
 }
}

Make your code into a function and, on the button, use

type="button"

and

onclick="yourFunction()"

You need to wrap your script into function and invoke it on button click:

<button onclick = "askForCar();">What do you drive?</button>

var askForCar = function() {
  //you script here
};

var askCar = function() {
  switch (prompt('What car?')) {
    case 'ferrari':
      document.write("you are probs a baller then");
      break;
    case "porsche":
      document.write("do you ball hard?");
      break;
    case "dodge":
      document.write("american cannot corner");
      break;
    default:
      document.write("your car better be a subaru");
  }
};

<button onclick="askCar();">What do you drive?</button>