Savings Goal Counter

What this page does

1. Type what you are saving for, what it costs, and how much you save each week.
2. Click the button. The page tells you how many weeks it takes.
3. The page also lists your total after every week.
4. Try typing 0 for the weekly amount. The page refuses. The next section explains why.

How it works

• saved is the accumulator. It starts at 0 and grows every week.
• weeks is the counter. It starts at 0 and adds 1 every week.
• while (saved < goal) runs the loop body again and again. The loop stops as soon as that condition is false.

let saved = 0;
let weeks = 0;

while (saved < goal) {
    saved = saved + perWeek;   // the accumulator grows
    weeks = weeks + 1;         // the counter grows
    log("Week " + weeks + ": $" + saved);
}

Build it step by step

1. 1

   Make the page

   Make index.html with the three inputs, the button, an #answer paragraph, and a #log area. The controls appear, but nothing happens yet.

2. 2

   Work it out on paper first

   Take a goal of 10 and a weekly amount of 4. Week 1 gives 4. Week 2 gives 8. Week 3 gives 12. That passes the goal, so you stop. You stopped when you reached the goal. That sentence is your while condition.

3. 3

   Write the loop

   In the click handler, read the boxes and turn them into numbers. Then write the while loop with saved, weeks, and one log line per week. Test it with the numbers from step 2. The page should match your paper.

4. 4

   See an infinite loop safely

   Work out what happens if you forget the line saved = saved + perWeek. Write down your answer first. Do not delete the line. Deleting it freezes the tab. Instead, run the safe version at the bottom of savings.js in the console. It uses a safety counter and break to stop the loop after 1000 passes.

   let safety = 0;
   while (saved < goal) {
       // the line that grows "saved" is missing on purpose
       weeks = weeks + 1;
   
       safety = safety + 1;
       if (safety > 1000) {
           console.log("Stopped. This loop was never going to end.");
           break;
       }
   }

5. 5

   Check the input

   Add an isNaN check so the page refuses text. Then add a check that refuses a weekly amount of 0 or less. That second check catches the infinite loop from step 4 before it can start.