A fibonacci sequence is written as:

0, 1, 1, 2, 3, 5, 8, 13, 21, ...

The Fibonacci sequence is the integer sequence where the first two terms are **0** and **1**. After that, the next term is defined as the sum of the previous two terms.

## Example 1: Fibonacci Series up to n terms

```
// program to generate fibonacci series up to n terms
// take input from the user
let number = parseInt(prompt('Enter the number of terms: '));
let n1 = 0, n2 = 1, nextTerm;
console.log('Fibonacci Series:');
for (let i = 1; i <= number; i++) {
console.log(n1);
nextTerm = n1 + n2;
n1 = n2;
n2 = nextTerm;
}
```

**Output**

Enter the number of terms: 4 Fibonacci Series: 0 1 1 2

In the above program, the user is prompted to enter the numbers of terms that they want in the Fibonacci series.

The `for`

loop iterates up to the number entered by the user.

**0** is printed at first. Then, in each iteration, the value of the second term is stored in variable `n1` and the sum of two previous terms is stored in variable `n2`.

## Example 2: Fibonacci Sequence Up to a Certain Number

```
// program to generate fibonacci series up to a certain number
// take input from the user
let number = parseInt(prompt('Enter a positive number: '));
let n1 = 0, n2 = 1, nextTerm;
console.log('Fibonacci Series:');
console.log(n1); // print 0
console.log(n2); // print 1
nextTerm = n1 + n2;
while (nextTerm <= number) {
// print the next term
console.log(nextTerm);
n1 = n2;
n2 = nextTerm;
nextTerm = n1 + n2;
}
```

**Output**

Enter a positive number: 5 Fibonacci Series: 0 1 1 2 3 5

In the above example, the user is prompted to enter a number up to which they want to print the Fibonacci series.

The first two terms **0** and **1** are displayed beforehand. Then, a `while`

loop is used to iterate over the terms to find the Fibonacci series up to the number entered by the user.