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JavaScript loops repeat a block of code so you can handle similar work across multiple steps or items without writing a separate copy of the instructions for every pass. The three basic forms are for, while and do...while: they differ in where they put the setup and update, and when they check whether to continue.
Why use loops in JavaScript?
Use a loop when the same kind of action needs to happen repeatedly—for example, drawing many circles or processing a list of items. Instead of duplicating the code for each pass, write the instructions once and let the loop repeat them.
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A loop continues while its condition says another pass should happen. For every loop, make sure something changes so that the condition can eventually become false; otherwise, the loop may run indefinitely.
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How does a for loop work?
A for loop keeps its initialization, condition and afterthought together in this form:
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for (initialization; condition; afterthought) {
// repeated instructions
}It is a clear choice when you know the counter and stopping rule together. In this example, the loop logs the numbers 0 through 4:
for (let i = 0; i < 5; i++) {
console.log(i);
}let i = 0runs once before the first pass and initializes the counter.i < 5is checked before each pass. If it is true, the body runs; if it is false, the loop ends.i++runs after each pass, increasing the counter before the next condition check.
Because the condition is checked before the body, a for loop can run zero times if its condition starts false.
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When should you use a while loop?
Use while when you want to repeat work as long as a condition remains true, especially when the stopping point is not simply a fixed count. It checks before each pass:
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while (attempts < 3) {
attempts++;
}Here, the body runs while attempts is less than 3. The increment changes the value on each pass, allowing the condition to become false. Since the test happens first, the body may run zero times if the condition is false from the outset.
When is do...while a better fit?
Choose do...while when an action needs to happen once before you decide whether it should repeat. It tests the condition after the body, so the body always runs at least once. A common pattern is to request input and then check whether it is valid:
let input;
do {
input = getInput();
} while (!isValid(input));getInput and isValid represent functions supplied by the application. The loop asks for input before checking whether another pass is needed.
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How do the three loop forms differ?
| Form | When the condition is checked | Can the body run zero times? | Introductory use |
|---|---|---|---|
for |
Before each pass | Yes | Known counter or range; setup, test and update stay together. |
while |
Before each pass | Yes | Repeat while a condition remains true. |
do...while |
After each pass | No | Perform an action once, then decide whether to repeat. |
Are these all the JavaScript loop types?
No. for, while and do...while are three basic control-flow forms, not the whole iteration toolkit. JavaScript also has for...of for iterable values such as array items, and for...in for enumerable object properties. When you want to process collection values and do not need their indexes, MDN recommends considering for...of.
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How can you avoid an infinite loop?
- Check that the condition can eventually become false.
- Make sure each pass updates a counter or otherwise changes the state used by the condition.
- For a
whileloop, do not forget to update that state inside the body when needed. - For a
do...whileloop, remember that the body runs once even if the condition would initially be false.
MDN’s guides explain loop structure and iteration in more detail: Loops and iteration and Loops and iteration in the JavaScript Guide.
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