JavaScript Program to Print Fibonacci Series

You can print the Fibonacci series in JavaScript by using a loop and two variables to store the last two numbers, like this: let a = 0, b = 1; for(let i = 0; i < n; i++) { console.log(a); [a, b] = [b, a + b]; }.

Examples

Input5

Output0 1 1 2 3

Input1

Output0

Input0

Output

How to Think About It

To print the Fibonacci series, start with the first two numbers 0 and 1. Then, keep adding the last two numbers to get the next one. Repeat this process until you have printed the desired count of numbers.

Algorithm

1

Get the number of terms to print (n).

2

Initialize two variables with 0 and 1 for the first two Fibonacci numbers.

3

Use a loop to repeat n times.

4

In each loop, print the current number.

5

Update the two variables to the next two Fibonacci numbers by adding them.

6

End the loop after printing n numbers.

Code

javascript

function printFibonacci(n) {
  let a = 0, b = 1;
  for (let i = 0; i < n; i++) {
    console.log(a);
    [a, b] = [b, a + b];
  }
}

printFibonacci(5);

Output

0
1
1
2
3

Dry Run

Let's trace printing 5 Fibonacci numbers through the code

1

Initialize

a = 0, b = 1

2

Iteration 1

Print a=0, update a=1, b=1

3

Iteration 2

Print a=1, update a=1, b=2

4

Iteration 3

Print a=1, update a=2, b=3

5

Iteration 4

Print a=2, update a=3, b=5

6

Iteration 5

Print a=3, update a=5, b=8

Iteration	a (printed)	b (next)
1	0	1
2	1	1
3	1	2
4	2	3
5	3	5

Why This Works

Step 1: Start with first two numbers

We begin with a = 0 and b = 1 because the Fibonacci series starts with 0 and 1.

Step 2: Print current number

In each loop, we print the current value of a which holds the current Fibonacci number.

Step 3: Update numbers for next iteration

We update a and b to the next two Fibonacci numbers by setting a = b and b = a + b (using the old values).

Alternative Approaches

Recursive function

javascript

function fibonacci(n) {
  if (n <= 1) return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

for (let i = 0; i < 5; i++) {
  console.log(fibonacci(i));
}

This method is simple but inefficient for large n because it recalculates values many times.

Using an array to store series

javascript

function printFibonacciArray(n) {
  const fib = [0, 1];
  for (let i = 2; i < n; i++) {
    fib[i] = fib[i - 1] + fib[i - 2];
  }
  for (let i = 0; i < n; i++) {
    console.log(fib[i]);
  }
}

printFibonacciArray(5);

This method stores all Fibonacci numbers, useful if you need to reuse them later but uses more memory.

Complexity: O(n) time, O(1) space

Time Complexity

The loop runs n times, so the time grows linearly with n.

Space Complexity

Only a few variables are used, so space stays constant regardless of n.

Which Approach is Fastest?

The iterative method is fastest and uses least memory compared to recursion or array storage.

Approach	Time	Space	Best For
Iterative loop	O(n)	O(1)	Fast and memory efficient
Recursive function	O(2^n)	O(n)	Simple but slow for large n
Array storage	O(n)	O(n)	When you need to reuse the series