How do I find out if Javascript really works?

In the planning phase (i.e. before writing code), you should have thought about what is your problem to solve and what requirements it poses. Test cases can be defined on this basis.

An example: A program for calculating the quotient from the division of two natural numbers (including 0) is to be developed.

A few possible test cases:

• 4 / 2 = 2
• 5 / 2 = 2.5
• 5.1 / 2 = not allowed
• 2 / 5.1 = not allowed
• 5 / 0 = NaN / not allowed
• 0 / 5 = 0
• a / 2 = NaN / not allowed
• 2 / a = NaN / not allowed

These cover not only the expected, usual applications, but also marginal cases.

Certainly you don’t always find all sorts of cases, but you can cover at least a large amount if you take some time. If you also do this before the concrete implementation, it can be easier and more stable than if you are not getting any changes in the aftermath.

During the implementation phase, you can implement unit tests that test individual functions. Yeast would be a framework that can be used in place.

A static code analysis would also be useful. If I only look at your codesnippet once, I’ll have undefined symbols: DisplayTimeSetting and Answer. With age again, you’re not doing anything.

Your used voice elements are no longer contemporary. Instead of variables with var to create, would let/const more sensible. You can send the request more easily with the Fetch API.

async function getAge() {
  const response = await fetch("update_time.php");

  if (!response.ok) {
    throw new Error("Request failed");
  }

  return await response.text();
}

async function updateTime() {
  const age = await getAge();
  // ...
}

In this case, I have divided the function into two functions, as the process Page is therefore simpler to test. If the function name fits, you should consider yourself.

For an analysis of the runtime behavior of the application, I would recommend using a debugger. In all the well-known browsers (Brave, Chrome, Edge, Firefox, Opera, Safari, …) one is integrated into their web development tools.