Run At Least Once
do...while runs the body first and checks the condition afterward.
Use it when the body must execute at least once.
JavaScript Tutorial
JavaScript do...while Loop
The do...while loop runs its body once before checking the condition.
This guarantees at least one iteration.
Why We Need It
Some tasks must run at least once, such as showing a menu or validating input.
do...while matches that pattern without extra boilerplate.
Syntax
do { ... } while (condition)Basic Example
1. Basic do...while
let i = 0;
do {
console.log(i);
i++;
} while (i < 3);The body runs before the condition check.
Real World Example
2. Run Once Even if False
let count = 0;
do {
console.log("Runs once");
} while (count > 0);Even though the condition is false, it runs once.
Multiple Use Cases
Validation Loops
It is useful for input validation and menus that must show at least once.
The loop continues as long as the condition remains true.
Avoid Infinite Loops
Just like while, make sure the condition can become false.
Update counters or state inside the loop.
More Examples
3. Menu Loop
let choice = "";
let attempts = 0;
do {
attempts++;
choice = attempts === 2 ? "exit" : "stay";
} while (choice !== "exit");
console.log(choice);Common for menu interactions and retries.
4. Validation Example
let value = -1;
do {
value++;
} while (value < 0);
console.log(value);Ensure a validation step happens at least once.
Comparison
Without
let i = 0;
console.log(i);
i++;
while (i < 1) {
console.log(i);
i++;
}With
let i = 0;
do {
console.log(i);
i++;
} while (i < 1);Common Mistakes and Fixes
Assuming the condition is checked first
Remember do...while checks after the body runs.
Forgetting to update the condition
Update counters or state to avoid infinite loops.
Using do...while unnecessarily
Use while if the body should not always run.
Interview Questions
When does do...while check the condition?
After running the body once.
When should you use do...while?
When you need the loop body to run at least once.
How do you avoid infinite loops?
Ensure the condition can become false.
Practice Problem
Practice: Use do...while to print numbers from 1 to 3.
let i = 1;
// TODO: print 1, 2, 3 using do...while
One Possible Solution
let i = 1;
do {
console.log(i);
i++;
} while (i <= 3);Frequently Asked Questions
What is the difference between while and do...while?
do...while runs the body at least once before checking the condition.
When should I use do...while?
When you need the body to run at least once, like menus or input validation.
Can do...while be replaced by while?
Yes, but you would need an extra first run before the loop.
Try It Yourself
Try a do...while loop and change the condition to see the effect.