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do...while - JavaScript | MDN

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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.

let result = "";
let i = 0;

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

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

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.

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.

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);

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);
  }
}

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.

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