Loops in JavaScript
Why Use Loops?
Loops let you run the same block of code multiple times—great for repeating tasks like processing items in a list, checking conditions, or building dynamic output.
for Loop
The for
loop is best when you know exactly how many times you want to repeat something.
Syntax:
for (initialization; condition; update) {
// code to run each loop
}
Example:
for (let i = 1; i <= 5; i++) {
console.log("Step", i);
}
Output:
Step 1
Step 2
Step 3
Step 4
Step 5
while Loop
Use while
when you want to repeat something as long as a condition is true.
Syntax:
while (condition) {
// code
}
Example:
jsCopyEditlet count = 1;
while (count <= 3) {
console.log("Count:", count);
count++;
}
do…while Loop
A do...while
loop is similar to while
, but it runs at least once, even if the condition is false at the start.
Syntax:
do {
// code
} while (condition);
Example:
let num = 1;
do {
console.log("Number:", num);
num++;
} while (num <= 2);
break and continue
break
Exits the loop completely when a certain condition is met.
for (let i = 1; i <= 5; i++) {
if (i === 3) break;
console.log(i);
}
// Output: 1 2
continue
Skips the current loop iteration and moves to the next one.
for (let i = 1; i <= 5; i++) {
if (i === 3) continue;
console.log(i);
}
// Output: 1 2 4 5
Looping Through Arrays
You’ll often loop over arrays to access their items.
Example with for
:
let fruits = ["apple", "banana", "cherry"];
for (let i = 0; i < fruits.length; i++) {
console.log(fruits[i]);
}
for…in vs for…of
Both are used to loop over collections, but they work differently.
for…in
- Loops over keys or indexes
- Use for objects or array indices
let student = { name: "Alice", age: 22 };
for (let key in student) {
console.log(key, ":", student[key]);
}
for…of
- Loops over values
- Works with arrays, strings, maps, sets
let colors = ["red", "green", "blue"];
for (let color of colors) {
console.log(color);
}
Summary
- Use
for
when you know the number of repetitions. - Use
while
for condition-based looping. - Use
do...while
when the loop should run at least once. break
exits the loop early;continue
skips to the next iteration.- Use
for...in
for object properties, andfor...of
for values in arrays or iterable items.