I am in the middle of converting a Polymer 2 app to Polymer 3. Modulizer did not work for me so I converted it manually. Thanks to the great upgrade guide it has been mostly straight forward so far.
One task is left though: in my Polymer 2 app I had a special html import (d3-import.html) that brought in the d3.js lib version 3 which comes as a plain JavaScript file (no ES6 module!). This import was dynamically loaded in only two out of overall 20 pages because the other 18 pages did not need it.
In Polymer 3 I can not import it as an ES6 module because it is not a module. Loading it in my main start.html would mean it gets loaded even if the user only uses the other 18 pages that don't need it. I tried writing script-tags in my web component templates but that doesn't seem to work. Unfortunately I don't see any error in the browser tools. The template simply stops to load at the line of the script-tags.
Any idea how to do this?
Additional question: since I start using lit-element in the same application. How to solve the same problem with lit-element?
Edit: note that I currently don't use any build steps/tools except for polymer-build to replace the module paths with actual file paths.
Note that this challenge has nothing to do with Polymer or LitElement, this is only an issue with how to load non-module resources from a module.
The most straightforward way that I know of is to use a bundler like Rollup that can support CommonJS or UMD. Rollup has the commonjs plugin for this: https://github.com/rollup/plugins/tree/master/packages/commonjs
The other option is to upgrade to D3 5.x, which appears to be published as standard modules itself. Given the number of files involved, you'll still likely want a bundler to reduce network roundtrips.