JavaScript code for recursive Fibonacci series

We have to write a recursive function fibonacci() that takes in a number n and returns an array with first n elements of fibonacci series. The Fibonacci sequence starts with 0 and 1, where each subsequent number is the sum of the two preceding ones.

Understanding the Fibonacci Sequence

The Fibonacci sequence follows this pattern: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34... where F(n) = F(n-1) + F(n-2).

Recursive Implementation

Here's a recursive approach that builds the Fibonacci array:

const fibonacci = (n, res = [], count = 1, last = 0) => {
    if(n){
        return fibonacci(n-1, res.concat(count), count+last, count);
    };
    return res;
};

console.log(fibonacci(8));
console.log(fibonacci(0));
console.log(fibonacci(1));
console.log(fibonacci(10));

[
  1, 1, 2, 3,
  5, 8, 13, 21
]
[]
[ 1 ]
[
  1, 1, 2, 3, 5,
  8, 13, 21, 34, 55
]

How It Works

The function uses four parameters:

• n: Number of elements remaining to generate
• res: Result array (starts empty)
• count: Current Fibonacci number (starts at 1)
• last: Previous Fibonacci number (starts at 0)

Alternative Simple Recursive Approach

Here's a more traditional recursive Fibonacci implementation:

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

// Generate array of first n Fibonacci numbers
function fibonacciArray(n) {
    const result = [];
    for (let i = 0; i < n; i++) {
        result.push(fibonacciSimple(i));
    }
    return result;
}

console.log(fibonacciArray(8));
console.log(fibonacciArray(5));

[
  0, 1, 1, 2,
  3, 5, 8, 13
]
[ 0, 1, 1, 2, 3 ]

Comparison

Conclusion

The tail recursive approach is more efficient for generating Fibonacci arrays, while the simple recursive method is easier to understand but has exponential time complexity for large inputs.