Add implementation with observer

Author: tshemsedinov

JavaScript/4-observer.js

@@ -0,0 +1,48 @@
+'use strict';
+
+const { EventEmitter } = require('events');
+const { max } = Math;
+
+const ee = new EventEmitter();
+
+const cities = [];
+
+const variables = {
+  maxDensity: 0,
+  count: 0,
+};
+
+ee.on('city', city => {
+  variables.count++;
+  variables.maxDensity = max(variables.maxDensity, city.density);
+  cities.push(city);
+  cities.forEach(city => {
+    city.relative = Math.round(city.density * 100 / variables.maxDensity);
+  });
+  console.table(variables);
+  console.table(cities);
+});
+
+ee.emit('city', {
+  city: 'Shanghai',
+  population: 24256800,
+  area: 6340,
+  density: 3826,
+  country: 'China',
+});
+
+ee.emit('city', {
+  city: 'Beijing',
+  population: 21516000,
+  area: 16411,
+  density: 1311,
+  country: 'China',
+});
+
+ee.emit('city', {
+  city: 'Delhi',
+  population: 16787941,
+  area: 1484,
+  density: 11313,
+  country: 'India',
+});