How to instantiate a javascript class in another js file?

It depends on what environment you’re running in. In a web browser you simply need to make sure that file1.js is loaded before file2.js:

<script src="https://stackoverflow.com/questions/12812757/file1.js"></script>
<script src="file2.js"></script>

In node.js, the recommended way is to make file1 a module then you can load it with the require function:

require('path/to/file1.js');

It’s also possible to use node’s module style in HTML using the require.js library.