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Introduction to JavaScript Runtime Environments

An introduction to the Node runtime environment and a browser's runtime environment.

What is a Runtime Environment?

A runtime environment is where your program will be executed. It determines what global objects your program can access and it can also impact how it runs. This article covers the two JavaScript runtime environments:

  1. the runtime environment of a browser (like Chrome, or Firefox)
  2. the Node runtime environment

A Browser’s Runtime Environment

The most common place where JavaScript code is executed is in a browser. For example, using any text editor, you could create a file on your own computer called my_website.html and put the following HTML code inside:

<!-- my_website.html --> <html> <body> <h1> My Website </h1> <script> window.alert('Hello World'); </script> </body> </html>

Save your file, then open your favorite browser. Most browsers will allow you to load websites that you have created locally by going to the menu File > Open File > my_website.html.

Upon loading, the embedded <script></script> will execute and the window.alert() method will create a pop-up box in your browser with the text "Hello World". How is this possible? Where did the window.alert() method come from and how can it control your browser?

The answer is that you are executing this code in the browser’s runtime environment. The window.alert() method is built into this environment and any program executed in a browser has access to this method. In fact, the window object provides access to a huge amount of data and functionality relating to the open browser window beyond just .alert().

Try replacing window.alert() with window.prompt() or window.confirm()

Applications created for and executed in the browser are known as front-end applications. For a long time, JavaScript code could only be executed in a browser and was used exclusively for creating front-end applications. In order to create back-end applications that could run on a computer WITHOUT a browser, you would need to use other programming languages such as Java or PHP.

The Node Runtime Environment

In 2009, the Node runtime environment was created for the purpose of executing JavaScript code without a browser, thus enabling programmers to create full-stack (front-end and back-end) applications using only the JavaScript language.

Node is an entirely different runtime environment, meaning that browser-environment data values and functions, like window.alert(), can’t be used. Instead, the Node runtime environment gives back-end applications access to a variety of features unavailable in a browser, such as access to the server’s file system, database, and network.

For example, suppose you created a file called my-app.js. We can check to see the directory that this file is located in using the Node runtime environment variable process:

// my-app.js console.log(process.env.PWD);

Notice that we are using console.log now instead of window.alert() since the window object isn’t available

process is an object containing data relating to the JavaScript file being executed. process.env is an object containing environment variables such as process.env.PWD which contains the current working directory (and stands for “Print Working Directory”).

To execute the JavaScript code in this file, first make sure that you have set up Node on your computer. Then, open up a terminal and run the following command:

$ node my-app.js /path/to/working/directory

The node command tells your computer to execute the my-app.js file in the Node environment. You can also use the node command without a file argument to open up the Node Read-Eval-Print-Loop (REPL):

$ node > process.env.HOME '/home/ccuser'


A runtime environment is where your program will be executed. JavaScript code may be executed in one of two runtime environments:

  1. a browser’s runtime environment
  2. the Node runtime environment

In each of these environments, different data values and functions are available, and these differences help distinguish front-end applications from back-end applications.

  • Front-end JavaScript applications are executed in a browser’s runtime environment and have access to the window object.
  • Back-end JavaScript applications are executed in the Node runtime environment and have access to the file system, databases, and networks attached to the server.

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