JavaScript while Loop

In the previous lesson, we learned how to repeat code a set number of times with a for loop. Now let’s meet the while loop, which keeps running as long as a condition stays true.

🔁 What is a while Loop?

A while loop runs a block of code over and over for as long as its condition is true. Before each pass, JavaScript checks the condition. If it is true, the loop runs the block once more. If it is false, the loop stops.

Here is the basic syntax:

while (condition) {
  // code that runs while the condition is true
}

The condition goes inside the parentheses, and the code we want to repeat goes inside the curly braces. As long as the condition evaluates to true, the block runs again.

🔢 Counting Up to 5

Let’s count from 1 up to 5 using a while loop.

let count = 1;

while (count <= 5) {
  console.log(count);
  count++; // update the condition variable
}
// 1
// 2
// 3
// 4
// 5

Let’s walk through what happens line by line:

• let count = 1 creates the variable we will track, starting at 1.
• while (count <= 5) checks the condition before each pass; while it is true, the block runs.
• console.log(count) prints the current value on each pass.
• count++ adds one to count, moving it closer to the stopping point.

This repeats until count becomes 6. At that point count <= 5 is false, so the loop stops.

🤔 When to Use while Instead of for

A for loop is the right choice when we know how many times we want to repeat. A while loop shines when we do not know the number of passes ahead of time and only want to keep going until some condition is met.

💰 Saving Until We Reach a Target

Let’s say Alex saves $20 each week and wants to know how many weeks it takes to reach $100. We do not know the answer in advance, so a while loop fits perfectly.

let savings = 0;
let weeks = 0;

while (savings < 100) {
  savings += 20; // add this week's deposit
  weeks++; // count the week
}

console.log(`It took ${weeks} weeks to reach $${savings}.`);
// It took 5 weeks to reach $100.

Let’s step through this loop:

• let savings = 0 and let weeks = 0 set our running total and week counter to zero before the loop starts.
• while (savings < 100) keeps the loop running until savings reaches the target.
• savings += 20 adds this week’s deposit on each pass.
• weeks++ counts the week so we know how long it took.
• console.log(...) prints the result once the loop ends.

Because we let the loop decide when to stop, we did not have to figure out the number of weeks ourselves.

♾️ Always Update the Condition Variable

The most important rule of a while loop is that something inside the block must change the condition so it eventually becomes false. If the condition never changes, the loop runs forever. This is called an infinite loop, and it will freeze the program.

// ❌ Wrong - count never changes, so this runs forever
let count = 1;
while (count <= 5) {
  console.log(count);
}

// ✅ Correct - count grows each pass, so the loop ends
let n = 1;
while (n <= 5) {
  console.log(n);
  n++;
}

Let’s compare the two versions:

• In the first loop, count stays at 1 forever because nothing updates it, so count <= 5 is always true and the loop never stops.
• In the second loop, n++ increases n on every pass, so n eventually reaches 6, the condition becomes false, and the loop ends.

The only difference is that one line that updates the variable.

⚠️ Common Mistakes to Avoid

🔧 Try It Yourself!

🧩 What You’ve Learned

• ✅ A while loop repeats code as long as its condition is true
• ✅ The syntax is while (condition) { ... }
• ✅ Use while when you do not know the number of passes ahead of time
• ✅ Something inside the block must update the condition variable
• ✅ Forgetting that update creates an infinite loop that freezes the program

🚀 What’s Next?

Next we will learn a loop that always runs at least once before checking its condition. Let’s continue to do while Loop.