## Development

### Using ES6 modules with node

node does not support ES6 modules at the moment of writing (2019-02-01).
However, John-David Dalton [1], the developer of lodash, made it possible
to use esm modules inside regular .js files with node, by making a 'userland'
solution [2].

This means that we can use esm modules (with import and export inside the 
javascript files) *and* we can run node on a set of unit tests directly,
without converting the code when doing that. For example:

$ node -r esm test/test.rect.js

Now works directly.

[1] https://github.com/jdalton
[2] https://github.com/standard-things/esm


### Producing a javascript bundle for use in a browser

As ES6 modules are not yet available in the browser, and many modules would
lead to overhead (download many files over a network connection), it is common
practice to make a bundle, i.e. one javascript file with content from all
modules.

One of the premises of ES6 modules is that (as we know completely beforehand
what content is available), we can only preserve in the bundle what we need
for running our javascript program.

Many tools exists in the javascript ecosystem that allow to bundle the
modules. In the process of bundling, it is also quite common to transpile
the javascript code to a slightly older version, so that more (older) browsers
are supported (even if the code in question uses new features/syntax).

One tool that can be used for making a javascript bundle is `rollup`. `rollup`
can furthermore be extended with plugins. `buble` is the plugin that transpiles
code from ES6 to ES5 (rewriting the new syntax into old syntax, understood by
slightly older browsers, that still exist out there).

In this project `rollup` is used. The project was started from the template [1].
[2] gives more information on why these templates were created.

[1] https://voorhoede.github.io/front-end-tooling-recipes/rollup-bundle-and-watch/
[2] https://www.voorhoede.nl/en/blog/fighting-front-end-fatigue-with-tooling-recipes/