Do-While Loop

Think about eating at a new restaurant. 🍛

You first taste the food, and then decide: "Is it good? Should I order more?"

You can't decide whether you like the food before tasting it. You must try it at least once.

In the last lesson, the while loop checked the condition first, and then ran the code. So if the condition was false at the start, the code never ran at all.

But sometimes we want the code to run at least once, no matter what, and then check the condition. For this, JavaScript gives us the do-while loop.

What Is a Do-While Loop?

A do-while loop runs the code first, and then checks the condition. If the condition is true, it repeats. So the code always runs at least once.

while loop: Check first, then run. 🤔 → 🏃
do-while loop: Run first, then check. 🏃 → 🤔

Syntax

do {
  // code to repeat
} while (condition);

Key points about the syntax:

The code block { } comes first, after the keyword do.
The while (condition) comes at the end.
Notice the semicolon ; after while (condition). It is required here, unlike in the normal while loop!

Your First Do-While Loop

How It Works, Step by Step

Step	Action	Value of `count`	Condition `count <= 5`
Round 1	Print "Count: 1", then `count++`	becomes `2`	✅ true → repeat
Round 2	Print "Count: 2", then `count++`	becomes `3`	✅ true → repeat
Round 3	Print "Count: 3", then `count++`	becomes `4`	✅ true → repeat
Round 4	Print "Count: 4", then `count++`	becomes `5`	✅ true → repeat
Round 5	Print "Count: 5", then `count++`	becomes `6`	❌ false → stop

The flow:

Run code → Check condition → true → Run code → Check condition → ...
                  ↓ (false)
              Exit the loop

The Big Difference: It Always Runs at Least Once

Let us see what happens when the condition is false from the very beginning.

With a while loop

The condition 10 < 5 is false, so the code never runs.

With a do-while loop

The code runs once first, and then JavaScript checks 10 < 5. It is false, so the loop stops. But the code already ran one time!

Remember: A do-while loop runs at least once, even if the condition is false from the start.

let i = 1;

do {
  console.log(i);
  // ❌ forgot i++ → runs forever!
} while (i <= 5);

How to avoid it:

✅ Always update the variable inside the loop.
✅ Make sure the update moves toward making the condition false.

While vs Do-While vs For

Point	For Loop	While Loop	Do-While Loop
Condition checked	Before each round	Before each round	After each round
Minimum runs	0	0	1
Best when	Number of repeats is known	Number of repeats is unknown	Code must run at least once
Semicolon at end?	No	No	Yes `while (...);`
Example	Print a table from 1 to 10	Save money until a goal	Show a menu, ask for a PIN

Simple way to remember:

"Do this 10 times" → for
"Keep doing this while..." → while
"Do this once, then repeat if needed" → do-while

Common Mistakes to Avoid

❌ Mistake 1: Forgetting the semicolon

do {
  console.log("Hello");
} while (false)    // ⚠️ semicolon missing

It often still works, but always add the semicolon to keep your code clean and safe:

} while (false);   // ✅

❌ Mistake 2: Using do-while when the code should not run

let cartItems = 0;

do {
  console.log("Processing payment...");   // ❌ runs even with an empty cart!
} while (cartItems > 0);

If the code should not run when the condition is false, use a normal while loop instead.

❌ Mistake 3: Declaring the variable inside the loop

do {
  let i = 1;   // ❌ resets every round
  i++;
} while (i <= 5);   // ❌ ReferenceError: i is not defined

Variables used in the condition must be created before the do block.

Quick Summary Table

Concept	Example
Basic do-while	`do { ... i++; } while (i <= 5);`
Runs once even if false	`do { ... } while (false);`
Repeat until the correct input	`do { ask(); } while (input !== correct);`
Menu loop	`do { showMenu(); } while (choice !== exit);`

Key Takeaways

A do-while loop runs the code first, then checks the condition.
It always runs at least once, even if the condition is false from the start.
It is perfect for menus, PIN or password entry, input validation, and games.
Don't forget the semicolon ; after while (condition).
Always update the loop variable to avoid infinite loops.

While Loop

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