For many Computer Science students, this generic programming problem will be familiar. However, a lot of front-end devs won’t have studied CS (I didn’t). I was asked to interview a CS undergraduate the other day and part of the process was to ask them to solve the following problem:

Write a function that returns the nth term of the Fibonacci sequence

My first question was… “What’s the Fibonacci sequence?”.

After determining what the problem was, I gave it a go myself. My solution is written in JS:

// function accepts the nth term as an argument
function fibonacci(n) {
    
    // validate argument
    if (!n || typeof(n) !== 'number') { return };

    // create an array and start the sequence
    var seq = [0,1];
                                        
    // determine how many iterations are required
    // generate the sequence up to the nth term only
    for (var i = 0; i < (n - 2); i++) {
        // calculate sum of last 2 terms in sequence
        var next = seq[seq.length - 1] + seq[seq.length - 2];
        // add nth term to sequence array
        seq.push(next);
    }
    
    // retrieve last number in sequence
    var x = seq[seq.length - 1];
    return x;
    
}

console.log(fibonacci(20)); // returns 4181​​​​​​​​​​​​​​​​​​​​​​​​​​

Download source files

There are recursive methods, which are more elegant but more expensive. I'm guessing there are also a thousand variations of the approach I took.

• Javascript
• Math

1 Comment

1. Lee says:

   December 19, 2012 at 14:57

   Very elegant solution