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A Mandelbot is a Mandelbrot image generator written in JavaScript. The goals include:

  • Make Mandelbots easy to use and configure (see Configuration)
  • Support multiple Mandelbots per page (see Demos)
  • Build it using clear, well-documented code (see mandelbot.js)
  • Experiment with third-party libraries (eg, the BigNumber JavaScript library)

However, this is a work-in-progress, and the Mandelbot feature set is not fully defined. The project started as a diversion and an excuse to learn some new things, so expect it to evolve.

For history buffs, I've archived an assortment of old related Pascal, C, and 8088 assembly-language code that I wrote over 30 years ago. And the original Scientific American article that helped spur a lot of early interest in the Mandelbrot Set (including my own) has been archived here as well.


This project has already been installed at the Mandelbot website using GitHub Pages and the gh-pages branch of this project. The website is constructed using Jekyll and a collection of Markdown documents.

If you want to install and serve your own local copies of all the files, here are the steps for macOS and the corresponding Terminal commands.

Step 1: Clone the project

git clone
cd mandelbot
git submodule update --init --recursive

Step 2: Install Homebrew (optional if you already have Ruby)

/usr/bin/ruby -e "$(curl -fsSL"

Step 3: Install Ruby (optional if you already have a sufficiently current version of Ruby)

brew install ruby

Step 4: Install Bundler

gem install bundler

Step 5: Install Jekyll (make sure your current directory is mandelbot, because this step requires the project's Gemfile)

bundle install

Step 6: Start the Jekyll server

bundle exec jekyll serve --host= --config _config.yml,_developer.yml

Step 7: Fire up your web browser and visit http://localhost:4000

In a perfect world, the above steps would be sufficient. Unfortunately, I ran into several problems along the way. Those were resolved by:

  • Updating Xcode to the latest version
  • Running xcode-select --install from a Terminal window

Ordinarily, the Jekyll server could be started with a simple bundle exec jekyll serve, but including the _developer.yml configuration file set the developer property for all pages, disabling these production-only features:

  • Google Analytics
  • Compiled ("minified") JavaScript

When running with uncompiled JavaScript, DEBUG is set to true, enabling console.log() operations, and support for a Debug button, which can be enabled for any Mandelbot that sets the idDebug property (see below). At the moment, the Debug control only displays the contents of the internal log (logDebug) in the page's status control, which is handy when the JavaScript console isn't available.

Notes for other operating systems (eg, Ubuntu)

On a clean Ubuntu-based system (elementary OS), I needed to:

sudo apt install git
sudo apt install ruby
sudo apt-get install ruby2.3-dev
sudo apt-get install zlib1g-dev
sudo gem install bundler
echo gem "'therubyracer'" >> Gemfile
bundle install
bundle exec jekyll serve --config _config.yml,_developer.yml

Without therubyracer added to the Gemfile, Jekyll would fail with the error "Could not find a JavaScript runtime."


Mandelbots are added to pages on the Jekyll-based Mandelbot website using the mandelbot.html include file:

{% include mandelbot.html id="mandelbot1" widthView="200" %}

For pages containing multiple Mandelbots, it may be more convenient to define each Mandelbot's configuration parameters at the top of the page, inside the page's Jekyll Front Matter:

  - id: mandelbot1
    widthView: 200

These "predefined" Mandelbots can then be added anywhere on the page, using just the id parameter:

{% include mandelbot.html id="mandelbot1" %}

Examples of both "inline" and "predefined" Mandelbots can be found on the demo page for Multiple Mandelbots.

There are also per-page properties that affect all Mandelbots on a page. For example, setting developer to true at the top of the page (inside the Front Matter):

developer: true

will load the uncompiled version of mandelbot.js, as shown on the demo page for Uncompiled Mandelbot.

Mandelbots support the following properties:

  • id: the unique id to use for the generated <canvas> element
  • widthView: the width of the view canvas, in pixels (default: 200)
  • heightView: the height of the view canvas, in pixels (default: widthView)
  • widthGrid: the width of the grid canvas on which numbers will plotted, in pixels (default: widthView)
  • heightGrid: the height of the grid canvas on which numbers will plotted, in pixels (default: heightView)
  • widthStyle: the width used to display the view canvas (default: auto)
  • heightStyle: the height used to display the view canvas (default: auto)
  • xCenter: the x coordinate of the center of the initial image (default: -0.65)
  • yCenter: the y coordinate of the center of the initial image (default: 0)
  • dxCenter: the distance from xCenter to the right and left sides of the initial image (default: 1.5)
  • dyCenter: the distance from yCenter to the top and bottom of the initial image (default: dxCenter)
  • bigNumbers: true to use BigNumbers for all floating-point calculations (default: false)
  • palette: one of the Mandelbot.PALETTE values (default: gray)
  • shape: one of the Mandelbot.SHAPE values (default: rect)
  • idStatus: a unique identifier for a text-based status control; if omitted, no control is generated
  • idDebug: a unique identifier for a Debug button control (available only when the developer property is true)
  • idReset: a unique identifier for a Reset button control; if omitted, no control is generated
  • idPrevious: a unique identifier for a Previous button control; if omitted, no control is generated

widthGrid and heightGrid determine the resolution of the image to be calculated, while widthView and heightView determine the resolution used to display that image on the page. They must be specified as numbers, and the units are pixels.

The grid is essentially an internal canvas representing the Cartesian coordinate grid onto which all the complex numbers are plotted, after they have passed through the Mandelbrot set calculations. The grid canvas is then drawn onto the view canvas. By default, the canvas sizes are the same, but different values can be used to create different aspect ratios, scaling effects, etc. The use of two canvases also provides automatic double-buffering, ensuring the smooth animation of current and future visual effects, such as the drawing of the selection rectangle.

widthStyle and heightStyle control how your browser displays the view canvas; they are simply passed through to the browser as standard CSS width and height properties on the canvas element using the style attribute. auto is the default for both properties. You can also specify numbers of pixels, but since these are CSS properties, you must also specify the units (eg, px). For example, a widthStyle of 200px enforces a display width of 200 pixels.

Generally, the only reason to alter the style settings is to make the view canvas responsive (ie, to fill the page as the width of the page changes). This is commonly done by setting widthStyle to 100%.

Some Mandelbot IDs are associated with special styles; see style.scss. For example, the Mandelbot on the home page uses ID "default", which has been given the following CSS properties:

#default {
    background-color: #ffffff;
    background-image: url(/assets/img/default.png);
    background-size: 100% auto;

Originally, this was done so that when other sites produced a thumbnail of the home page, they would (hopefully) pick up the default image that the Mandelbot initially produces. However, some sites (like Facebook) also require adding some metadata to the default page template:

<meta property="og:image" content="">


Visit the website.

Default Mandelbot


Google's Closure Compiler is used to create minified, ES5-compatible JavaScript files.

Both the original and minified JavaScript source files are checked into the project, so this may be of little interest unless you plan to modify the JavaScript files, in which case you can install the Closure Compiler and other assorted development tools listed in package.json using the Node Package Manager (which, of course, requires that you have Node installed):

npm install

Once Node and NPM have been installed, and npm install has successfully completed, the provided gulpfile can be used to invoke the compiler:

npm run compiler


Scientific American, August 1985: Exploring the Mandelbrot Set

"Computer Recreations: A computer microscope zooms in for a look at the most complex object in mathematics" by A. K. Dewdney, pp. 16-24.


Copyright © 2017-2019 Jeff Parsons. This is an open source project with no formal license. All portions not licensed from other sources may be freely reused. Any derivative work just needs to provide attribution along with the above copyright.

Portions copyright 2012 Christian Stigen Larsen and licensed under Apache License, Version 2.0. Those portions are clearly identified in mandelbot.js and must be accompanied by the same Apache License if they are redistributed.