Using Window.prompt() within a JS function (more Mad Libs analogies)

Published on January 22, 2021

Look at this variation of the function I referred to in my last post:

function applyDiscount(cost, discount) {
  const eligible = prompt("Are you eligible for the discount? (y/n)");
  if (eligible.toLowerCase() === "y") {
    const newCost = cost - (cost * discount);
    return newCost;
  return cost;

We fixed the redundant declaration const discount = 0.25 so now our function works properly. And we moved eligible from being a function argument; it is now a const that is declared within the function, but one which is calling the Window.prompt() method to request and receive user input.

This works... there's nothing wrong with it functionally. But for the purposes of teaching new programmers JavaScript, I think it is needlessly confusing. Why? Because it is combining two different approaches to passing values into a function... and that's a technique that new programmers just don't need to try to grasp yet. Let's go back to the Mad Libs analogy.

Here, you fill out this blank yourself...

Mad Libs cartoon

Our Mad Libs fans are at it again, filling out a new page to make yet another ridiculous and nonsensical story. Person A is holding the book and asking Person B for words:

"Give me a noun."


"An adjective."


But then Person A hands Person B the book and asks: "You write the next word right here on this blank."


That still works... they're not doing anything wrong. Person B is still providing a word for all of the blanks in the story... but the flow and continuity are disrupted, and for what reason?

Arguments vs. variables

A beginning programmer has a lot of new concepts to wrestle with: variables hold values, functions manipulate data, arguments are passed into functions, etc. When a JavaScript function receives arguments, the basic assumption is: the values of these arguments can vary each time the function is called, just like the Mad Libs blanks. It's the stuff within the function that remains static and will be used to transform the arguments into a unique result.

But when we put one variable inside of a function that then calls a nested function (the Window.prompt() method) to grab a value at runtime... we risk giving our new programmers migraines for no reason.

So let's keep our example functions for our newbies simple. If a value is going to vary at runtime, then make it an argument to the function. Here's the original function, simplified:

function applyDiscount(eligible, cost, discount) {
  if (eligible === "y") {
    const newCost = cost - (cost * discount);
    return newCost;
  return cost;

const eligible = prompt("Are you eligible for the discount? (y/n)").toLowerCase();
const cost = 24;
const discount = 0.25;
console.log(applyDiscount(eligible, cost, discount));

And now we have access to the value of eligible outside the scope of the function... just like we have access to the values used for the cost and discount.

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