Fibonacci in Javascript (original) (raw)

  1. Home
  2. Programming Projects in Every Language
  3. Fibonacci
  4. Fibonacci in Javascript

Published on 04 October 2018 (Updated: 04 May 2025)

Fibonacci in Javascript

Welcome to the Fibonacci in Javascript page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

function fibonacci(num) {
    let n = Number(num);
    let first = 0;
    let second = 1;
    let result = 0;
    for (let i = 1; i <= n; i++) {
        result = first + second;
        first = second;
        second = result;
        console.log(i + ": " + first);
    }
}

num = process.argv[2];

if (num && !isNaN(num)) {
    fibonacci(num);
} else {
    console.log("Usage: please input the count of fibonacci numbers to output");
}

Fibonacci in Javascript was written by:

This article was written by:

If you see anything you'd like to change or update, please consider contributing.

Note: The solution shown above is the current solution in the Sample Programs repository as of May 04 2025 12:16:22. The solution was first committed on Oct 04 2018 13:17:58. The documentation was last updated on May 15 2023 15:51:23. As a result, documentation below may be outdated.

How to Implement the Solution

Let's look at the code in detail:

Here we have a function called fibonacci that takes in a numeric value as an argument that corresponds to the amount Fibonacci numbers we want to print in succession.

What fibonacci" does is that it starts printing from 1 then each time it just prints the accumulation of the last number it printed (stored in variable named second) and the 2nd last number it printed (stored in variable named first).

Then we have a variable named num which can have a numeric value of 10 since we want to print the first 10 numbers in the Fibonacci sequence. I can also have the value process.argv[2] so we can run the command node fibonacci.js 10 to execute the file with NodeJS and print the first 10 numbers in the Fibonacci sequence.

Then we have a function that verifies that num has a positive, numeric value so we can run the function named fibonacci, else it just returns an instruction/warning.

How to Run the Solution

If you want to run this code in a browser

  1. Copy/paste the code provided in a JavaScript file.
  2. Give the variable num a value of 10 instead of process.argv[2].
  3. Link the script in a web page. (example given below)

For example: If you copy/paste this code in a file named fibonacci.js then use copy/paste the following tag in your HTML file:

<script src="fibonacci.js"></script>
  1. Also make sure that fibonacci.js is in the same folder/directory as your HTML file.
  2. Just open the webpage in a web browser and look at it's console. The output will be there.

If you want to run this code with NodeJS

  1. Make sure that the variable num has a value of `process.argv[2]".
  2. Open the Terminal/CMD and move to the directory where you are keeping fibonacci.js.
  3. Execute the command node fibonacci.js 10 to execute the file with NodeJS to print the first 10 numbers in the Fibonacci sequence.