Loops in JavaScript
for, while, do-while explained

Understanding Loops in JavaScript

Loops allow us to execute a block of code repeatedly. This is useful when you want to run the same code multiple times with different values or conditions.

JavaScript provides several types of loops. In this lesson, we will cover the most common ones:

• for loop
• while loop
• do-while loop

The for Loop

The for loop is best when you know beforehand how many times you want to run the loop.

for (let i = 1; i <= 5; i++) {
  console.log("Iteration:", i);
}
    

Output:

Iteration: 1
Iteration: 2
Iteration: 3
Iteration: 4
Iteration: 5
    

Explanation:

• let i = 1 → loop starts at 1
• i <= 5 → runs while condition is true
• i++ → increases i by 1 on each loop

Q: What happens if we change i <= 5 to i < 5?

A: The loop will run from 1 to 4, not 1 to 5. Because i = 5 is not less than 5.

The while Loop

The while loop is useful when you don’t know in advance how many times you need to loop.

let count = 1;

while (count <= 3) {
  console.log("Count is", count);
  count++;
}
    

Output:

Count is 1
Count is 2
Count is 3
    

Explanation: The while loop checks the condition before each run. If count <= 3 is true, it enters the loop.

Q: What happens if we forget count++?

A: The loop becomes infinite because the condition count <= 3 will always be true. This is a common beginner mistake.

The do-while Loop

The do-while loop is similar to while, but it runs at least once before checking the condition.

let x = 6;

do {
  console.log("x is", x);
  x++;
} while (x <= 5);
    

Output:

x is 6
    

Explanation: Even though x <= 5 is false from the start, the loop body executes once before the condition is checked.

Q: When should you use do-while instead of while?

A: Use do-while when you need the code to run at least once regardless of the condition.

Loop Comparison Table

Loop Type	Condition Checked	Runs at least once?	Best Use Case
for	Before loop body	No	Known number of iterations
while	Before loop body	No	Unknown number of iterations
do-while	After loop body	Yes	Need to run at least once

Conclusion

JavaScript loops are essential for automating repetitive tasks. Start with for loops, move to while for more dynamic conditions, and use do-while when an action must be guaranteed at least once.

In the next lesson, we’ll explore functions in JavaScript and how they help structure your code efficiently.