JavaScript Mapping in 2019

It's been five-and-a-half years since I last blogged about making maps in a browser using JavaScript. It's cool that my old Syriac Places demo still works. Yay for web standards. A lot of things have changed in the meanwhile and this is my first chance to acknowledge those changes.

I've been writing JavaScript every other day at work for the past 3 weeks, building a web app to give Sales and sales-supporting teams at Mapbox insight into our imagery basemap. It's HTML and mostly Vanilla JS and has been a fun exercise. Since it's for internal customers only I can use new browser and JavaScript features.

Mapbox GL JS, the JavaScript mapping library that's maintained by my coworkers, is the closest thing to a framework in my app. My current project is the first in which I've used Mapbox GL JS. I'd like to point out some features and aspects of the framework that I find especially notable.

Mapbox GL JS has excellent examples. This is a result of I cribbed freely from and everything worked. Not all projects have good examples. John Firebaugh (who has since moved on from Mapbox), Colleen McGinnis, and the GL JS and Documentation teams put a ton of work into these and it shows.

Mapbox GL JS makes a big and important case of dynamic maps simple. I have a GeoJSON Source defined in my app's JavaScript. It is registered with a layer and a map. When I remove features from the source, they disappear from the map. When I add features to the source, they appear on the map. The map is updated fast enough that I can do this on every mousemove event.

Mapbox GL JS Expressions give a developer ample control over styling and filtering while keeping the styling API abstract and simple. In several parts of its API, Mapbox GL JS can evaluate Lisp-like expressions composed of JavaScript arrays. ["+", 4, ["*", 3, 2]] evaluates to 4 + (3 * 2), or 10. In addition to the standard JavaScript operators, the namespace of GL JS expressions operators contains feature property and map attribute getters. An advantages of using JavaScript objects instead of conventional Lisp (as with snuggs and rio-calc) is that developers get syntax highlighting help when writing expressions, and can use JavaScript to build expressions.

Another first for me is that I'm using Turf, the well-known simple features geometry library created by Morgan Herlocker. In my app I'm creating features on the fly derived from the intersections of features in a Mapbox tileset. Turf is fast and its functions take and return GeoJSON objects, which makes it easy for me to use, and easy to integrate with Mapbox GL JS and other libraries or plugins. Turf is indispensible for a web app like the one I'm writing.

Lastly, JavaScript itself has changed a lot in the last 5 years. I'm using features of ES6 for the first time ever. Arrow functions are like Scheme's lambda expressions and Python's lambda forms. I'm using them exclusively, instead of the old anonymous functions, when filtering and sorting arrays of features. For example, the following sorts features by a date, in descending date order, without needing to type "function" or "return". Handy.

features.sort((a, b) => -