The JavaScript do...while loop is a control flow statement that creates a loop that executes a block of code at least once, and then repeatedly executes the block, or not, depending on a given boolean condition at the end of the block. This structure is ideal when you need to perform an action at least one time and then continue looping based on a condition.
Example 1: Basic Counter
// Initialize a counter variable
let i = 0;
do {
// Print the value of i to the console
console.log(`The value of i is: ${i}`);
// Increment the counter
i++;
} while (i < 5); // The loop continues as long as i is less than 5
Explanation
This code initializes a variable i to 0. The do...while loop then prints the current value of i and increments it. The condition i < 5 is checked after the first execution, and the loop continues until i is no longer less than 5.
Example 2: User Input Validation
let input;
do {
// Prompt the user for input
input = prompt("Please enter a number greater than 10:");
} while (isNaN(input) || input <= 10); // Continue prompting if the input is not a number or is not greater than 10
console.log("You entered a valid number:", input);
Explanation
This example demonstrates how to use a do...while loop for input validation. It will continuously prompt the user to enter a number until they provide a value that is a number and is greater than 10, ensuring valid input is received.
Example 3: Menu Selection
let choice;
do {
// Display a menu and get the user's choice
choice = prompt("Select an option:\n1. Start Game\n2. Options\n3. Exit");
} while (choice !== "3"); // Continue showing the menu until the user chooses to exit
console.log("Exiting the menu.");
Explanation
This code snippet showcases a simple menu system. The do...while loop ensures the menu is displayed at least once and will keep reappearing until the user enters "3" to exit the menu.
Example 4: Simulating a Dice Roll
let diceRoll;
do {
// Generate a random number between 1 and 6
diceRoll = Math.floor(Math.random() * 6) + 1;
// Log the result of the roll
console.log(`You rolled a ${diceRoll}`);
} while (diceRoll !== 6); // The loop continues until a 6 is rolled
console.log("You rolled a 6!");
Explanation
This example simulates rolling a die. The do...while loop guarantees the die is rolled at least once and continues to "roll" until the result is a 6.
Example 5: Countdown Timer
let count = 5;
do {
// Display the current count
console.log(`Countdown: ${count}`);
// Decrement the count
count--;
} while (count >= 0); // Continue counting down as long as the count is non-negative
console.log("Liftoff!");
Explanation
This code implements a simple countdown timer. The do...while loop starts at 5 and counts down to 0, printing each number along the way before finally printing "Liftoff!".
Example 6: Fetching Data Until Success
let dataFetched = false;
let attempts = 0;
do {
attempts++;
console.log(`Attempt ${attempts}: Fetching data...`);
// In a real scenario, an actual data fetching operation would be here.
// We'll simulate a potential failure and success.
if (Math.random() > 0.5) {
dataFetched = true;
console.log("Data fetched successfully!");
} else {
console.log("Failed to fetch data, retrying...");
}
} while (!dataFetched); // Continue trying to fetch data until it is successful
Explanation
This example simulates an attempt to fetch data that might fail. The do...while loop ensures the fetch operation is attempted at least once and will retry until the dataFetched variable becomes true.
Example 7: Array Manipulation
const numbers = [10, 20, 30, 40, 50];
let i = 0;
do {
// Double the value of the current element in the array
numbers[i] *= 2;
// Move to the next index
i++;
} while (i < numbers.length); // Continue as long as there are elements in the array
console.log("Modified array:", numbers); // Output will be [20, 40, 60, 80, 100]
Explanation
In this final example, we manipulate an array. The do...while loop iterates through the numbers array, doubling the value of each element.