JavaScript runs on most modern browsers, as web developers, we use this to enhance the user’s experience. Java applets can also run in a browser, but they have been losing the popularity contest that JavaScript is taking over since the emergence of mobile devices and browsers. Some important reasons for this are compatibility and security, which as developers we try to avoid these at all cost. They can also both run in the server, where Java has thrived for years with a large portion web applications using Java. JavaScript has closed the gap in this area with the rise of JavaScript-powered application servers, with this the popularity of Node.js took-off. Both also have a massive amount of frameworks and libraries to help speed up development.
One of the biggest differences between the two programming languages is that Java code is typically written in an Integrated Development Environment (IDE) and compiled into bytecode. Because humans can’t read bytecode we have the Java Virtual Machine (JVM) to run it. JavaScript code is normally executed by a JavaScript engine in the same syntax in which it is written, although JavaScript files that are sent over the internet are often compressed into a barely readable format to reduce their size. Making changes in Java application environments can require several steps using specialized software to compile and deploy the changes while making changes to JavaScript can sometimes be done with just a simple text editor.