A choropleth map of Singapore

A basic choropleth map of Sinagpore, showing the mean resale price per square meter for houses in different planning areas.

The (async () => { ... })() bit is a so-called Immediately Invoked Function Expression, which is just a way to define a function and run it immediately without having to give it a name. It is used here to contain all logic that depends on asynchronous operations like fetching datasets.