JavaScript `do...while` Loop: A Beginner's Guide

7.4 The do...while Loop

In the world of programming, loops are essential tools that allow us to execute a block of code repeatedly. In JavaScript, we have several types of loops, each with its unique characteristics and use cases. In this section, we will delve into the do...while loop, a loop that ensures a block of code is executed at least once before checking a condition. This feature sets it apart from other loops like the while loop.

Understanding the do...while Loop

The do...while loop is a control flow statement that executes a block of code once and then repeats the execution as long as a specified condition evaluates to true. The key feature of the do...while loop is that it guarantees the code block inside the loop will run at least once, regardless of whether the condition is true or false.

Syntax of the do...while Loop

The syntax of the do...while loop is straightforward:

do {
    // Code to be executed
} while (condition);

• do: This keyword indicates the start of the loop.
• Code Block: The code inside the curly braces {} is executed once before the condition is tested.
• while: This keyword is followed by a condition that is evaluated after the code block is executed.
• Condition: If the condition is true, the loop will execute the code block again. If false, the loop will terminate.

How the do...while Loop Differs from the while Loop

The primary difference between the do...while loop and the while loop lies in when the condition is evaluated:

• while Loop: The condition is checked before the code block is executed. If the condition is false initially, the code block may never run.

  let count = 0;
  while (count < 5) {
      console.log(count);
      count++;
  }
  

• do...while Loop: The code block is executed first, and then the condition is checked. This guarantees that the code block runs at least once.

  let count = 0;
  do {
      console.log(count);
      count++;
  } while (count < 5);
  

Practical Use Cases for do...while Loops

The do...while loop is particularly useful in scenarios where you need to ensure that a block of code runs at least once. Here are some practical use cases:

1. User Input Validation: When prompting a user for input, you may want to ensure that the user provides a valid input before proceeding. The do...while loop can be used to repeatedly prompt the user until a valid input is received.

2. Menu Selection: In console applications, you might display a menu and require the user to make a selection. The do...while loop can ensure that the menu is displayed at least once and continues to display until a valid choice is made.

3. Retry Mechanisms: In situations where you need to retry an operation until it succeeds, such as network requests or file operations, the do...while loop can be employed to attempt the operation at least once and continue if necessary.

Example: User Input Validation

Let’s consider a scenario where we need to prompt a user to enter a number between 1 and 10. We can use a do...while loop to ensure that the prompt is displayed at least once and continues until a valid number is entered.

let number;
do {
    number = parseInt(prompt("Enter a number between 1 and 10: "), 10);
} while (isNaN(number) || number < 1 || number > 10);

console.log(`You entered: ${number}`);

In this example:

• The prompt function is used to display a dialog box that asks the user to enter a number.
• The parseInt function converts the input string to an integer.
• The isNaN function checks if the input is not a number.
• The loop continues until the user enters a valid number between 1 and 10.

Example: Menu Selection

Consider a simple console application that displays a menu and requires the user to make a selection. The do...while loop ensures that the menu is displayed at least once and continues until the user makes a valid choice.

let choice;
do {
    console.log("Menu:");
    console.log("1. Start Game");
    console.log("2. View Scores");
    console.log("3. Exit");

    choice = parseInt(prompt("Enter your choice (1-3): "), 10);
} while (isNaN(choice) || choice < 1 || choice > 3);

switch (choice) {
    case 1:
        console.log("Starting game...");
        break;
    case 2:
        console.log("Viewing scores...");
        break;
    case 3:
        console.log("Exiting...");
        break;
}

In this example:

• The menu is displayed using console.log.
• The user is prompted to enter a choice.
• The loop continues until the user enters a valid choice (1, 2, or 3).
• A switch statement is used to handle the user’s choice.

Example: Retry Mechanism

In scenarios where an operation may fail and needs to be retried, such as fetching data from a server, the do...while loop can be used to attempt the operation at least once and continue if necessary.

let success = false;
let attempts = 0;
const maxAttempts = 3;

do {
    attempts++;
    console.log(`Attempt ${attempts}: Fetching data...`);

    // Simulate a fetch operation with a 50% chance of success
    success = Math.random() > 0.5;

    if (success) {
        console.log("Data fetched successfully!");
    } else {
        console.log("Failed to fetch data. Retrying...");
    }
} while (!success && attempts < maxAttempts);

if (!success) {
    console.log("Failed to fetch data after maximum attempts.");
}

In this example:

• A success variable is used to track whether the operation succeeded.
• A maxAttempts variable limits the number of retries.
• The loop continues until the operation succeeds or the maximum number of attempts is reached.
• The Math.random function simulates a 50% chance of success.

Visualizing the do...while Loop

To better understand the flow of the do...while loop, let’s visualize it using a flowchart:

    flowchart TD
	    A[Start] --> B[Execute Code Block]
	    B --> C{Condition True?}
	    C -->|Yes| B
	    C -->|No| D[End]

In this flowchart:

• The loop starts by executing the code block.
• After executing the code block, the condition is checked.
• If the condition is true, the loop repeats the code block.
• If the condition is false, the loop ends.

Try It Yourself

Experimenting with code is a great way to solidify your understanding. Try modifying the examples above to see how the do...while loop behaves in different scenarios. Here are some ideas:

• Modify the user input validation example to prompt for a different range of numbers.
• Add more menu options to the menu selection example.
• Change the retry mechanism example to simulate different success rates.

Conclusion

The do...while loop is a valuable tool in JavaScript programming, especially when you need to ensure that a block of code runs at least once. By understanding its unique characteristics and practical use cases, you can effectively incorporate it into your programs. Remember to experiment with the examples provided and explore additional resources to deepen your understanding.

Further Reading

For more information on the do...while loop and other JavaScript loops, consider exploring the following resources:

• MDN Web Docs: do…while
• W3Schools: JavaScript Loops

Quiz Time!