Types of loops in JS
1. for: the classic loop
Used when the number of iterations is known in advance.
javascript
for (let i = 0; i < 5; i++) {
console.log(i);
}Result:
javascript
0
1
2
3
4Structure:
javascript
for (initialization; condition; step) {
// loop body
}initializationruns once at the startconditionis checked before every iterationstepruns after every iteration
2. while: a pre-condition loop
Runs while the condition is true.
javascript
let i = 0;
while (i < 5) {
console.log(i);
i++;
}Used when the number of iterations is not known in advance.
3. do...while: a post-condition loop
Guaranteed to run at least once.
javascript
let i = 5;
do {
console.log(i);
i++;
} while (i < 5);Result:
javascript
5The loop body runs first, then the condition is checked.
4. for...in: iterating over an object's keys
Used to walk through an object's properties.
javascript
const user = { name: 'Oleh', age: 25, city: 'Kyiv' };
for (let key in user) {
console.log(key, ':', user[key]);
}Result:
javascript
name : Oleh
age : 25
city : KyivFeatures:
- It iterates over all enumerable properties, including inherited ones.
- It does not guarantee order.
- It is better not to use it for arrays.
5. for...of: iterating over iterable objects
Ideal for arrays, strings, Set, Map, and so on.
javascript
const numbers = [10, 20, 30];
for (let num of numbers) {
console.log(num);
}Result:
javascript
10
20
30It also works with a string:
javascript
for (let char of 'JS') {
console.log(char);
}It iterates over values, not keys (unlike
for...in).
6. forEach(): an array method
Not an operator, but an array method, though it does the same job: iteration.
javascript
const arr = ['a', 'b', 'c'];
arr.forEach((value, index) => {
console.log(index, value);
});Result:
javascript
0 a
1 b
2 cFeatures:
- You cannot use
breakorcontinue. - It is convenient for iterations without an early exit.
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