do...while
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
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 thedo...while
loop.
Description
Like other looping statements, you can use control flow statements inside statement
:
break
stopsstatement
execution and goes to the first statement after the loop.continue
stopsstatement
execution and re-evaluatescondition
.
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
Specification |
---|
ECMAScript Language Specification # sec-do-while-statement |
Browser compatibility
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