for & while Loops

A loop repeats a block. A for loop has a starting step, a condition checked before each pass, and an update after each pass. In for (let day = 1; day <= 3; day = day + 1), the block runs for 1, 2, and 3. At 4 the condition is false.

An accumulator holds a result across passes. Initialize it before the loop. Putting let total = 0 inside would erase the previous work each time. total += 20 is a shorter way to write total = total + 20; learn the long form first so you know what it means.

A while loop checks a condition before each repetition too, but you write the update in its body. Every loop needs a way to finish. If you accidentally remove the update and it keeps running, use the editor’s Stop control. Do not test deliberately endless loops.

Predict before running

Write a small table with columns day and total. What is total after the second pass? Change the target from three days to four.

let total = 0;

for (let day = 1; day <= 3; day = day + 1) {
  total = total + 20;
}

console.log(total);

60

Trace a changing total

A trace table is a debugging tool you can make on paper. Record the counter and total after each pass. This makes it easier to distinguish a wrong update from a wrong stopping condition.

The update day = day + 1 runs after the body. The condition runs again before the next body. Once day is 4, the body is skipped and execution continues after the loop.

let total = 0;

for (let day = 1; day <= 3; day = day + 1) {
  total = total + day;
  console.log(`${day}: ${total}`);
}

console.log(`Final: ${total}`);

1: 1
2: 3
3: 6
Final: 6

Choose the loop from the question

Use a counting for loop when the number of steps is known. Use while when a condition describes unfinished work. Neither choice removes your responsibility to update the state correctly. Increasing a counter while checking whether it stays above zero can move away from termination.

Fix on paper first: a countdown starts at 3, checks remaining > 0, and adds one each time. Replace the update before running so it moves toward zero. Then decide whether zero should be printed: that requirement changes where you place the print and what comparison you use.

A useful habit is to test zero iterations, one iteration, and several iterations. Starting with an already-satisfied goal should not accidentally add one more session.

Reading a while loop

The shape is while (condition) { instructions }: check the condition, run the instructions if it is true, then check again. Unlike the three-part for-loop header, there is no automatic update slot. Put the update inside the body. In your exercise, total < 40 is the question and adding 10 moves total toward the stopping point.

Trace the test, body and update separately

A for loop does not run all three parts on every line. Initialization happens once; the condition is checked before each body; the update happens after a completed iteration. When the condition is false initially, the body never runs. This is why an accumulator needs a useful value before entering the loop.

A while loop uses the same repeated test, but you place the update in its body. Write a reason the condition will eventually become false before running it. Avoid experimenting with an unbounded loop just to see what happens: the useful skill is predicting termination.

let total = 0;

for (let day = 1; day <= 3; day = day + 1) {
  console.log(day);
  total = total + 10;
}

console.log(total);

1
2
3
30

The <= comparison includes day 3. After its body, the update makes day 4, and the next check stops the loop. Replacing <= with < changes both the number of iterations and the total.

let savings = 60;
let days = 0;

while (savings < 50) {
  savings = savings + 15;
  days = days + 1;
}

console.log(days);
console.log(savings);

0
60

The goal is already met before the first test. The correct answer is zero new saving days. Initializing days to one would bake an iteration into the result even though none happened.

Try the rule on new inputs

For the saving loop, try starting amounts 0, 49 and 50. Record the condition before each iteration and the amount afterward. Then translate the bounded for example into a while loop with the same outputs.

Your turn

let total = 0;
// Repeat while total is below 40.

let total = 0;

while (total < 40) {
  total = total + 10;
  console.log(total);
}

10
20
30
40

What you should be able to explain

• Describe initialization, condition, body, and update in their actual execution order.
• What happens if the condition is false before the first pass?
• Make a trace table for adding 10 until a total reaches 30.