Do While Loop in JavaScript: Syntax, Examples and Best Practices

let count = 1;

do {
  console.log("Count:", count);
  count++;
} while (count <= 5);

// Output:
// Count: 1
// Count: 2
// Count: 3
// Count: 4
// Count: 5

do {
  // code to repeat
  // runs at least once — ALWAYS!
} while (condition);

let x = 100; // Condition will be false immediately

// while — checks condition FIRST, body never runs
while (x < 5) {
  console.log("while body ran"); // Never prints
}

// do...while — runs body FIRST, then checks condition
do {
  console.log("do...while body ran"); // Prints ONCE ✅
} while (x < 5);

// Output: do...while body ran

let i = 1;

do {
  console.log(i);
  i++;
} while (i <= 4);

// Scenario: condition is FALSE from the very start

let n = 10;

// while — body SKIPPED entirely
while (n < 5) {
  console.log("while ran"); // Never executes
}

// do...while — body runs ONCE regardless
do {
  console.log("do...while ran"); // Runs once ✅
} while (n < 5);

// Simulated user choices
let choices = [1, 2, 3, 4]; // 1=View, 2=Add, 3=Delete, 4=Exit
let step = 0;
let choice;

do {
  choice = choices[step++];

  console.log("\n📋 MAIN MENU");
  console.log("1. View Items");
  console.log("2. Add Item");
  console.log("3. Delete Item");
  console.log("4. Exit");
  console.log(`\nYou chose: ${choice}`);

  switch (choice) {
    case 1: console.log("📄 Showing all items..."); break;
    case 2: console.log("➕ Adding new item..."); break;
    case 3: console.log("🗑️ Deleting item..."); break;
    case 4: console.log("👋 Goodbye!"); break;
    default: console.log("❌ Invalid choice. Try again.");
  }

} while (choice !== 4);

// Output: Menu displays all 4 times, exits on choice 4

// Simulated inputs — first two invalid, third valid
let inputs = [-5, 0, 42];
let index = 0;
let userInput;

do {
  userInput = inputs[index++];
  console.log(`Input received: ${userInput}`);

  if (userInput <= 0) {
    console.log("❌ Invalid! Please enter a positive number.\n");
  }
} while (userInput <= 0);

console.log(`✅ Valid input accepted: ${userInput}`);
// Output:
// Input received: -5
// ❌ Invalid! Please enter a positive number.
//
// Input received: 0
// ❌ Invalid! Please enter a positive number.
//
// Input received: 42
// ✅ Valid input accepted: 42

const correctOtp = "7291";
const otpAttempts = ["1234", "0000", "7291"]; // Simulated inputs
let otpIndex = 0;
let enteredOtp;
let maxTries = 3;
let tries = 0;

do {
  enteredOtp = otpAttempts[otpIndex++];
  tries++;
  console.log(`OTP Attempt ${tries}: ${enteredOtp}`);

  if (enteredOtp === correctOtp) {
    console.log("✅ OTP Verified! Login successful.");
    break;
  } else {
    console.log(`❌ Wrong OTP. ${maxTries - tries} attempt(s) remaining.`);
  }
} while (tries < maxTries);

if (enteredOtp !== correctOtp) {
  console.log("🔒 Account locked. Please request a new OTP.");
}
// Output:
// OTP Attempt 1: 1234
// ❌ Wrong OTP. 2 attempt(s) remaining.
// OTP Attempt 2: 0000
// ❌ Wrong OTP. 1 attempt(s) remaining.
// OTP Attempt 3: 7291
// ✅ OTP Verified! Login successful.

// Simulate rolling a dice until we get a 6
let roll;
let rollCount = 0;
let rolls = [2, 5, 1, 4, 3, 6]; // Simulated dice rolls
let rollIndex = 0;

do {
  roll = rolls[rollIndex++];
  rollCount++;
  console.log(`Roll ${rollCount}: 🎲 ${roll}`);
} while (roll !== 6);

console.log(`\n🎉 Rolled a 6 after ${rollCount} attempt(s)!`);
// Output:
// Roll 1: 🎲 2
// Roll 2: 🎲 5
// Roll 3: 🎲 1
// Roll 4: 🎲 4
// Roll 5: 🎲 3
// Roll 6: 🎲 6
// 🎉 Rolled a 6 after 6 attempt(s)!

// Retry a failed download up to 3 times
let downloadSuccess = false;
let attempt = 0;
let maxAttempts = 3;

// Simulate: first two fail, third succeeds
let results = [false, false, true];

do {
  attempt++;
  downloadSuccess = results[attempt - 1];

  console.log(`📥 Download attempt ${attempt}...`);

  if (downloadSuccess) {
    console.log("✅ Download successful!");
  } else {
    console.log(`❌ Failed. Retrying in ${attempt * 2}s...`);
  }
} while (!downloadSuccess && attempt < maxAttempts);

if (!downloadSuccess) {
  console.log("🚫 Download failed after 3 attempts. Please try again later.");
}
// Output:
// 📥 Download attempt 1...
// ❌ Failed. Retrying in 2s...
// 📥 Download attempt 2...
// ❌ Failed. Retrying in 4s...
// 📥 Download attempt 3...
// ✅ Download successful!

// Keep generating numbers until we hit a prime
let num = 20;

do {
  num++;
  let isPrime = true;

  for (let i = 2; i < num; i++) {
    if (num % i === 0) {
      isPrime = false;
      break;
    }
  }

  if (isPrime) {
    console.log(`First prime after 20: ${num}`);
    break; // Exit do...while as soon as we find it
  }
} while (num < 100);
// Output: First prime after 20: 23

// Process a queue — skip already-processed items
let queue = ["task-1", "DONE", "task-2", "DONE", "task-3"];
let qIndex = 0;

do {
  let task = queue[qIndex];
  qIndex++;

  if (task === "DONE") {
    console.log(`⏭️  Skipping completed task`);
    continue;
  }

  console.log(`⚙️  Processing: ${task}`);
} while (qIndex < queue.length);

// Output:
// ⚙️  Processing: task-1
// ⏭️  Skipping completed task
// ⚙️  Processing: task-2
// ⏭️  Skipping completed task
// ⚙️  Processing: task-3

// ❌ SyntaxError — missing semicolon at the end
do {
  console.log("hello");
} while (false)  // ← missing ;

// ✅ Semicolon is required
do {
  console.log("hello");
} while (false); // ✅

let i = 1;

// ❌ i never changes — infinite loop!
do {
  console.log(i);
  // forgot i++
} while (i <= 5);

// ✅ Always update inside the body
do {
  console.log(i);
  i++; // ✅
} while (i <= 5);

let isAuthenticated = false;

// ❌ do...while runs the protected code ONCE even when not authenticated!
do {
  console.log("🔓 Loading dashboard..."); // Runs even though user isn't logged in!
} while (isAuthenticated);

// ✅ Use while — checks condition BEFORE running
while (isAuthenticated) {
  console.log("🔓 Loading dashboard..."); // Correctly skipped ✅
}

let i = 1;

// ❌ When i === 3, continue jumps back BEFORE i++ — loops forever on 3!
do {
  if (i === 3) continue; // i never increments past 3!
  console.log(i);
  i++;
} while (i <= 5);

// ✅ Always increment BEFORE continue
do {
  if (i === 3) {
    i++; // ✅ Update first
    continue;
  }
  console.log(i);
  i++;
} while (i <= 5);
// Output: 1  2  4  5

let score = 0;

// ❌ You expect this to be skipped — but it runs once!
do {
  console.log("Game over screen shown"); // Runs even with score 0
} while (score > 100);

// ✅ If the body should be conditional from start, use while
while (score > 100) {
  console.log("Game over screen shown"); // Correctly skipped ✅
}

// 🎯 Do...While Loop Practice

// 1. Number guessing game simulation
console.log("=== 🎯 Number Guessing Game ===");
const target = 47;
const guesses = [10, 70, 40, 55, 47];
let gIndex = 0;
let guess;
let gAttempts = 0;

do {
  guess = guesses[gIndex++];
  gAttempts++;

  if (guess < target)      console.log(`Guess ${gAttempts}: ${guess} — Too low ⬆️`);
  else if (guess > target) console.log(`Guess ${gAttempts}: ${guess} — Too high ⬇️`);
  else                     console.log(`Guess ${gAttempts}: ${guess} — 🎉 Correct!`);

} while (guess !== target && gIndex < guesses.length);

console.log(`Solved in ${gAttempts} attempt(s)!\n`);


// 2. ATM PIN entry simulation
console.log("=== 🏧 ATM PIN Entry ===");
const correctPin = "9876";
const pinAttempts = ["1111", "0000", "9876"];
let pinIndex = 0;
let pin;
let pinTries = 0;
const maxPinTries = 3;

do {
  pin = pinAttempts[pinIndex++];
  pinTries++;

  if (pin === correctPin) {
    console.log(`✅ PIN accepted on attempt ${pinTries}. Welcome!`);
  } else {
    console.log(`❌ Wrong PIN (Attempt ${pinTries}/${maxPinTries})`);
  }
} while (pin !== correctPin && pinTries < maxPinTries);

if (pin !== correctPin) {
  console.log("🔒 Card blocked after 3 wrong attempts.\n");
}


// 3. Survey Form Simulation
console.log("=== 📋 Survey Form ===");
const questions = [
  { q: "How satisfied are you? (1-5)",  answer: 4 },
  { q: "Would you recommend us? (1-5)", answer: 5 },
  { q: "Rate our support (1-5)",        answer: 3 },
];

let qIndex = 0;
let totalRating = 0;

do {
  let current = questions[qIndex];
  totalRating += current.answer;
  console.log(`Q${qIndex + 1}: ${current.q}`);
  console.log(`Answer: ${current.answer}/5\n`);
  qIndex++;
} while (qIndex < questions.length);

let avgRating = (totalRating / questions.length).toFixed(1);
console.log(`📊 Average Rating: ${avgRating}/5`);
console.log(avgRating >= 4 ? "🌟 Excellent feedback!" : "📝 Room for improvement.");

node do-while-loop.js

=== 🎯 Number Guessing Game ===
Guess 1: 10 — Too low ⬆️
Guess 2: 70 — Too high ⬇️
Guess 3: 40 — Too low ⬆️
Guess 4: 55 — Too high ⬇️
Guess 5: 47 — 🎉 Correct!
Solved in 5 attempt(s)!

=== 🏧 ATM PIN Entry ===
❌ Wrong PIN (Attempt 1/3)
❌ Wrong PIN (Attempt 2/3)
✅ PIN accepted on attempt 3. Welcome!

=== 📋 Survey Form ===
Q1: How satisfied are you? (1-5)
Answer: 4/5

Q2: Would you recommend us? (1-5)
Answer: 5/5

Q3: Rate our support (1-5)
Answer: 3/5

📊 Average Rating: 4.0/5
🌟 Excellent feedback!