Fibonacci Sequence Adder Game in Javascript. Got bored while doing homework so I wrote this in ~10 minutes. F12 on your browser, copy the code, paste it in that window that shows up (called a Developer Console) and play along!

fibonacci.js

// Function scoped - prevent global namespace pollution
(function (window) {
    'use strict';
    
    // Bootstraps our Fibonacci program
    function runProgram() {
        
        /*
            Recursive function with two base cases: 
                First 2 terms of Fibonacci sequence are 0 and 1.
                The next terms after those terms are the sum of the
                    two previous terms.
        */
        function fibonacci(termsToAdd) {
            if (termsToAdd === 1) {
                return 0;
            } else if (termsToAdd === 2) {
                return 1;
            }
            return fibonacci(termsToAdd - 1) + fibonacci(termsToAdd - 2);
        }

        // By default, input is 0 unless the user puts something in
        var input
            = Number(window.prompt("How many Fibonacci Terms to add up?")) || 0,
            playAgain;

        // Protect against invalid input
        if (window.isNaN(input)) {
            window.alert("Inappropriate input - input must be a number!");
        } else {
            window.alert("The sum of Fibonacci sequence of the first "
                               + input + " term/s is " + fibonacci(input));
        }
        
        playAgain
            = window.prompt("Play Again? Yes or No?").toLowerCase() || "yes";
        
        // If you notice, this is a double recursion program... sneaky sneaky
        if (playAgain === "yes") {
            runProgram();
        }
        
    }
    
    // Run our Fibonacci program!
    runProgram();
    
}(window));

c00l. but it doesn't output the sum of the first input n terms. it outputs the number at position n in the sequence.