do...while Statement

The do...while statement creates a loop that executes a specified statement as long as the test condition evaluates to true. The condition is evaluated after executing the statement, resulting in the specified statement executing at least once.

A do…while loop in JavaScript is a control statement in which the code is allowed to execute continuously based on a given boolean condition. It is like a repeating if statement.

The do…while loop can be used to execute a specific block of code at least once.

Types of Loops

There are mainly two types of loops:

1. Entry Controlled Loops: In this type of loop, the test condition is tested before entering the loop body. For Loop and While Loops are entry-controlled loops.
2. Exit Controlled Loops: In this type of loop, the test condition is tested or evaluated at the end of the loop body. Therefore, the loop body will execute at least once, irrespective of whether the test condition is true or false. The do...while loop is an exit controlled loop.

Example

let result = '';
let i = 0;

do {
  i = i + 1;
  result = result + i;
} while (i < 5);

console.log(result);
// Expected output: "12345"

Output:

"12345"

Syntax

do
  statement
while (condition);

• statement: A statement that is executed at least once and re-executed as long as the condition evaluates to true. You can use a block statement to execute multiple statements.
• condition: An expression evaluated after each pass through the loop. If this condition evaluates to true, the statement is re-executed. When the condition evaluates to false, execution continues with the statement after the do...while loop.

Description

Like other looping statements, you can use control flow statements inside the statement:

• break stops statement execution and goes to the first statement after the loop.
• continue stops statement execution and re-evaluates the condition.

The do...while statement syntax requires a semicolon at the end, but the automatic semicolon insertion process may insert one for you if the lack of a semicolon results in invalid syntax.

Examples

Using do...while

In the following example, the do...while loop iterates at least once and reiterates until i is no longer less than 5.

let result = "";
let i = 0;
do {
  i += 1;
  result += `${i} `;
} while (i > 0 && i < 5);

console.log(result);
// Despite i === 0 this will still loop as it starts off without the test

Using false as do...while Condition

Because the statement is always executed once, do...while (false) is the same as executing the statement itself. This is a common idiom in C-like languages, which allows you to use break to break out of branching logic early.

do {
  if (!user.loggedIn) {
    console.log("You are not logged in");
    break;
  }
  const friends = user.getFriends();
  if (!friends.length) {
    console.log("No friends found");
    break;
  }
  for (const friend of friends) {
    handleFriend(friend);
  }
} while (false);
// The rest of the code

In JavaScript, there are some alternatives, such as using a labeled block statement with break:

handleFriends: {
  if (!user.loggedIn) {
    console.log("You are not logged in");
    break handleFriends;
  }
  const friends = user.getFriends();
  if (!friends.length) {
    console.log("No friends found");
    break handleFriends;
  }
  for (const friend of friends) {
    handleFriend(friend);
  }
}

Or using a function:

function handleFriends() {
  if (!user.loggedIn) {
    console.log("You are not logged in");
    return;
  }
  const friends = user.getFriends();
  if (!friends.length) {
    console.log("No friends found");
    return;
  }
  for (const friend of friends) {
    handleFriend(friend);
  }
}

Using an Assignment as a Condition

In some cases, it can make sense to use an assignment as a condition, such as this:

do {
  // …
} while ((match = regexp.exec(str)));

But when you do, there are readability trade-offs. The while documentation has a Using an assignment as a condition section with recommendations.

Note

If you use a variable in the condition, you must initialize it before the loop, and increment it within the loop. Otherwise, the loop will never end. This will crash your browser. If the condition is always true, the loop will never end. This will also crash your browser.

Parameters

• condition (Required): The condition for running the code block. If true, the loop will start over again, otherwise it ends.