Selective error catching in JavaScript

selective_error_catch.js

// from: Eloquent JavaScript http://eloquentjavascript.net/08_error.html#p_CJKevweHV6

// Define our own error type
function InputError(message) {
  this.message = message;
  this.stack = (new Error()).stack;
}
InputError.prototype = Object.create(Error.prototype);
InputError.prototype.name = "InputError";

// Check for instances of that error type and rethrow if needed
function promptDirection(question) {
  var result = prompt(question, "");
  if (result.toLowerCase() == "left") return "L";
  if (result.toLowerCase() == "right") return "R";
  throw new InputError("Invalid direction: " + result);
}

for (;;) { // Loops until explicitly broken
  try {
    var dir = promptDirection("Where?");
    console.log("You chose ", dir);
    break;
  } catch (e) {
    if (e instanceof InputError)
      console.log("Not a valid direction. Try again.");
    else
      throw e;
  }
}