How to Create a Javascript Program
The browser on your desktop is a computing engine with a well-designed user interface. Behind the interactive environment, it can read local HTML files and execute programs written in javascript. This page is a step-by-step guide to working in that enviroment, with examples and links to get you started with using it as a research tool.
Begin with Python
In order to get started you will need a local web server. Since most likely you will not have one running that is easily accessible, a simple solution is to use python to provide one for you. Python has a built-in module that will run a server and that can be started with one line of python code.
First, identify the directory or folder on your computer under which all of the javascript you want to access is stored. Then, at the top level of that directory, run this program
python -m CGIHTTPServer 8000 1>/dev/null 2>/dev/null &
By using port 8000 the server is distinct from the one on port 80 used for web applications. The site would appear by putting
http://localhost:8000
in a Google Chrome or Mozilla Firefox browser window running on the same user account on the same machine. Note the redirects for stdio and stderr to /dev/null keeps output from appearing in the console. The server may be killed by identifying its process ID in Linux with the command
ps -e | grep python
followed by
kill -s 9 pid
where "pid" is the ID number found in the first line. Alternatively, if it is the only python process running you may kill it with
killall python
Any file in the directory tree below the starting directory is now accessible in the browser, and html files will be parsed to run the included javascript. If here is a cgi-bin directory at the top level, the server will see it and use it. One use of this low level server is to create a virtual instrument that is accessible from the web, but not exposed to it directly. A remote web server on the same network that can access port 8000 on the instrument machine can run code and get response from the instrument by calling cgi-bin operations.
For programmers, however, this utility allows development and debugging of web software without the need for a large server.
Add a Directory for a Program
Put each program (think webpage) into its own directory under this top level directory. That way, when you open the browser to localhost:8000 you will see list of directories each containing a separate application. Click on one of those and your application, or its files, will open.
Under that program directory create at a minimum two other subdirectories:
js will contain the javascript code css will contain styling code if you want to implement it
Your javascript programs will have the extension ".js" and should be located in the js subdirectory. This structure is optional, but keeping to the pattern will make writing and rewriting programs easier later.
The browser will automatically see a file "index.html" in a directory if that file exists. For development it is best not to use "index.html" as the base file for your application, but instead to use another descriptive name such as "calculator.html" or "orbits.html". Later, if you want it to open without the extra click you can rename it, or in Linux and other Unix-like operating systems create a soft link that points to it with the command line
ln -s calculator.html index.html
The browser will see the index soft link and then open the file it points to. By not having "index.html" in your program directory you will be able to browse other files there, try variants on your program, or inspect code in the browser.
Select a Browser
There are two browsers that are recommended for this type of development, and the choice at least at this beginning stage is a matter of style and convenience. The user interface to development help is different, and actually the "engine" they run is also different. As the web develops, the programmers who create and maintain browsers also change what they can do, and sometimes what they allow the browsers to do.