A TypeScript do...while loop runs its body once before checking its condition, then repeats while that condition is true. Use it when the first operation must happen at least once—for example, when prompting for input before deciding whether to ask again.
Do-while syntax in TypeScript
Write the loop with do, a block of statements, and while followed by a condition. End the construct with a semicolon:
do {
// Statements to run at least once
} while (condition);The syntax is JavaScript syntax and can be used in TypeScript. MDN’s do…while reference documents the construct. This explains the loop’s language behavior, not any version-specific TypeScript compiler diagnostics or configuration.
How a do-while loop runs
- Run the statements inside the body.
- Evaluate the condition after the body finishes.
- If the condition is true, run the body again. If it is false, continue with the statement after the loop.
Since the condition is checked after the first pass, even a condition that is false from the start does not skip the body. That is the defining difference from a pre-test while loop, which checks its condition before entering the body and can therefore run zero times. See MDN’s do…while reference for the post-test behavior.
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Example: prompt until input is valid
A do-while loop is useful when an action must happen once before the program can decide whether to repeat it:
let attempts = 0;
let isValid = false;
do {
attempts += 1;
isValid = checkInput();
} while (!isValid && attempts < 3);This calls checkInput() on the first pass. It repeats only if the input is still invalid and fewer than three attempts have been made. The attempt counter changes on each pass, so the limit can eventually make the condition false.
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Choose between do-while and while
| Loop | When condition is checked | Minimum body executions | Choose it when |
|---|---|---|---|
do...while |
After the body | One | The first operation must happen before the repetition decision. |
while |
Before the body | Zero | The body should run only if the condition is already true. |
For straightforward counted iteration, a for loop may be clearer. To visit items without needing an index, consider for...of. These are readability choices, not TypeScript restrictions.
Use break and continue
break exits the loop immediately. continue skips the rest of the current body and proceeds to the condition check; if the condition is true, another pass begins. MDN describes these controls in its do…while reference.
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let value = 0;
do {
value += 1;
if (shouldStop(value)) {
break;
}
if (shouldSkip(value)) {
continue;
}
process(value);
} while (value < 10);In this example, break can end the loop before its condition is tested. When continue runs, the loop still checks value < 10 before deciding whether to repeat.
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Avoid common do-while mistakes
- Expecting a false condition to skip the body: the body runs before the first condition check, so it always gets one pass unless control exits from inside it.
- Leaving out the semicolon: use the conventional form
} while (condition);. MDN documents the terminating semicolon in the syntax. - Forgetting a way to stop: check that the body changes state used by the condition, or that a deliberate
breakpath can exit. If neither happens and the condition remains true, the loop can continue indefinitely. - Using it when zero passes are valid: choose a pre-test loop instead if the condition should be able to prevent the first execution.
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