Loops allow you to execute code repeatedly based on a condition.
1️⃣ Basic for Loop
👉 Example:
for (let i = 1; i <= 5; i++) {
console.log(i);
}
💡 Output:
1 2 3 4 5
2️⃣ Loop with if Condition
👉 Example: Print even numbers
for (let i = 1; i <= 10; i++) {
if (i % 2 === 0) {
console.log(i);
}
}
💡 Output:
2 4 6 8 10
3️⃣ Nested for Loop
👉 Example: Pattern (matrix)
for (let i = 1; i <= 3; i++) {
for (let j = 1; j <= 3; j++) {
console.log(`Row ${i}, Column ${j}`);
}
}
💡 Concept:
- Outer loop → rows
- Inner loop → columns
4️⃣ while Loop
👉 Example:
let i = 1;while (i <= 5) {
console.log(i);
i++;
}
💡 Use case:
- When iteration count is unknown
5️⃣ do...while Loop
👉 Example:
let i = 1;do {
console.log(i);
i++;
} while (i <= 5);
💡 Key Point:
- Executes at least once
6️⃣ forEach Loop (Array Only)
👉 Example:
let arr = [10, 20, 30];arr.forEach((value, index) => {
console.log(index, value);
});
7️⃣ Loop with String Datatype
A string is iterable, so you can loop through characters.
👉 Example using for
let str = "Hello";for (let i = 0; i < str.length; i++) {
console.log(str[i]);
}
👉 Example using for...of
for (let char of "Hello") {
console.log(char);
}
💡 Output:
H e l l o
8️⃣ Loop with Array Datatype
👉 Example using for
let numbers = [1, 2, 3, 4];for (let i = 0; i < numbers.length; i++) {
console.log(numbers[i]);
}
👉 Example using for...of
for (let num of numbers) {
console.log(num);
}
👉 Example using forEach
numbers.forEach(num => console.log(num));
9️⃣ Loop with Object Datatype
Objects are not directly iterable like arrays, but we use for...in.
👉 Example:
let person = {
name: "John",
age: 25,
city: "Delhi"
};for (let key in person) {
console.log(key, person[key]);
}
💡 Output:
name John
age 25
city Delhi
🔥 Summary Table
| Loop Type | Best Use Case |
|---|---|
for | Fixed iterations |
while | Unknown iterations |
do...while | Runs at least once |
forEach | Arrays |
for...of | Arrays & strings |
for...in | Objects |
Common Mistakes
❌ Infinite Loop
for (let i = 1; i <= 5; ) {
// missing increment
}
❌ Wrong Object Loop
for (let item of person) // ❌ Error
✔ Correct:
for (let key in person)
✅ Practice Ideas
- Reverse a string using loop
- Find largest number in array
- Count vowels in string
- Print object keys and values
- Create star pattern using nested loop