JavaScript Loops


Loop statements in JavaScript allow you to repeat a block of code multiple times until a specified condition is met. They are essential for automating repetitive tasks and iterating through data structures like arrays or objects. JavaScript supports several types of loop statements:

1. For Loop

The for loop is the most commonly used loop when the number of iterations is known beforehand. It runs a block of code a specific number of times.

Syntax


for (initialization; condition; increment) {
    // code to be executed
}

  • Initialization: Usually used to declare and initialize a counter variable.
  • Condition: The loop continues to run as long as this condition evaluates to true.
  • Increment/Decrement: Updates the counter variable after each iteration.


Example

for (let i = 0; i < 5; i++) {
    console.log(“Iteration number ” + i);
}

Output

Iteration number 0
Iteration number 1
Iteration number 2
Iteration number 3
Iteration number 4



2. While Loop

The while loop repeats a block of code as long as the specified condition remains true. It is often used when the number of iterations isn’t known in advance.

Syntax

while (condition) {
    // code to be executed
}

Example

let i = 0;
while (i < 5) {
    console.log(“Iteration number “ + i);
    i++;
}
 
Output

 Iteration number 0
Iteration number 1
Iteration number 2
Iteration number 3
Iteration number 4


3. do…whileLoop

The do…while loop is similar to the while loop, but it guarantees that the code block will be executed at least once, even if the condition is false initially. The condition is checked after the block of code is executed.

Syntax

do {
    // code to be executed
} while (condition);


Example

let i = 0;

do {
    console.log(“Iteration number ” + i);
    i++;
} while (i < 5); 

Output

Iteration number 0
Iteration number 1
Iteration number 2
Iteration number 3
Iteration number 4


4. for…in Loop

The for…in loop iterates over the properties (or keys) of an object. Objects in JavaScript are collections of key-value pairs, where each key (also called a property name) is associated with a value. The for…in loop allows you to access these keys one by one.

Syntax

for (variable in object) {
    // code to be executed
}

Example

let person = {name: “Alice”, age: 25, city: “New York”};
 
for (let key in person) {
    console.log(key + “: ” + person[key]);
}
 

Output

 name: Alice
age: 25
city: New York


5. for…of Loop

The for…of loop is used to iterate over iterable objects like arrays, strings, maps, or sets. It gives the values of the iterable, rather than its keys.

Syntax

for (variable of iterable) {
    // code to be executed
}

Example

let numbers = [1, 2, 3, 4, 5];

for (let number of numbers) {
    console.log(number);
}

Output

1
2
3
4
5


Important tips

  • The for loop is used to execute a block of code a specific number of times.
  • The while loop executes a block of code as long as a specified condition is true.
  • The do…while loop is similar to the while loop, but it will always execute the block of code at least once before checking the condition.
  • The for…in loop is used to iterate over the properties of an object.
  • The for…of loop is used to iterate over iterable objects (like arrays, strings, maps, sets, etc.).


 

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