Learning D3.js 5 Mapping - Second Edition

Drawing the background scene

Before we draw the airports, we manipulate the airport locations data. We create an array holding one GeoJSON point geometry object per airport:

var airportLocation = [];airports.forEach(function(el) {  var obj = {};  obj.type = 'Feature';  obj.id = el.iata;  obj.geometry = {    type: 'Point',    coordinates: [+el.long, +el.lat]  };  obj.properties = {};  airportLocation.push(obj);});airportMap = d3.map(airportLocation, function(d) { return d.id; });

Then we fill the global airportMap variable with a map we produce with d3.map(). d3.map() is a utility function that takes an array of objects to produce key-value pairs we can access with its own map.get() method. We won't use the map immediately but will get to it in a moment. ...