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.

Try it

let result = "";
let i = 0;

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

console.log(result);
// Expected 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, statement is re-executed. When 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 statement:

• break stops statement execution and goes to the first statement after the loop.
• continue stops statement execution and re-evaluates 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);
// Despite i === 0 this will still loop as it starts off without the test

console.log(result);

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 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 tradeoffs. The while documentation has a Using an assignment as a condition section with our recommendations.

Specifications

Browser compatibility

See also

• while
• for
• break
• continue