do...while

¥Syntax

do
  statement
while (condition);

statement

至少执行一次的语句，只要条件计算为真，就会重新执行。你可以使用 块语句 来执行多个语句。

condition

每次循环后计算的表达式。如果此条件 评估结果为真，则重新执行 statement。当条件 评估结果为假 时，继续执行 do...while 循环之后的语句。

¥Description

与其他循环语句一样，你可以在 statement 内使用 控制流语句：

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

• break 停止 statement 执行并转到循环后的第一个语句。
• continue 停止 statement 执行并重新评估 condition。

do...while 语句语法要求末尾有一个分号，但如果缺少分号导致语法无效，自动插入分号 进程可能会为你插入一个分号。

¥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

在以下示例中，do...while 循环至少迭代一次并重复，直到 i 不再小于 5。

¥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

因为该语句总是执行一次，所以 do...while (false) 与执行语句本身相同。这是 C 类语言中的常见习语，它允许你使用 break 尽早摆脱分支逻辑。

¥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

在 JavaScript 中，有一些替代方案，例如将 标记块语句 与 break 一起使用：

¥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);
  }
}

或者使用函数：

¥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

在某些情况下，使用赋值作为条件是有意义的，例如：

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

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

但当你这样做时，就需要牺牲可读性。while 文档有一个 使用赋值作为条件 部分，其中包含我们的建议。

¥But when you do, there are readability tradeoffs. The while documentation has a Using an assignment as a condition section with our recommendations.

¥See also

• while
• for
• break
• continue