I created a kotlin multiplatform library then created a number of kotlin/js classes. How can I use the compiled kotlin/js code (module) in a different frontend application.

I use multiplatform plugin, but for kotlin/js should be the same: You could generate a node library by using

jsNodeDevelopmentLibraryDistribution

gradle task and then import this library into another node project by declaring it as a dependency in package.json:

{
...
  "dependencies": {
    "mylib": "file:../build/developmentLibrary",
    ...
  }
}

Its for the browser.