Add lense example

tshemsedinov · tshemsedinov · commit 26b07b897ff9 · 2018-10-17T02:35:58.000+03:00
diff --git a/JavaScript/5-lense.js b/JavaScript/5-lense.js
@@ -0,0 +1,58 @@
+'use strict';
+
+// Lens
+
+const view = (lens, obj) => lens.get(obj);
+
+const lens = (source, destination = source) => ({
+  get: obj => obj[source],
+  set: (val, obj) => ({ ...obj, [destination]: val })
+});
+
+const id = x => x;
+
+const field = (name, map = id) => obj => map(view(lens(name), obj));
+
+// Projection
+
+const projection = meta => {
+  const keys = Object.keys(meta);
+  return obj => {
+    const hash = {};
+    keys.forEach(key => {
+      const field = meta[key];
+      hash[key] = field(obj);
+    });
+    return hash;
+  };
+};
+
+// Dataset
+
+const persons = [
+  { name: 'Marcus Aurelius', city: 'Rome', born: 121 },
+  { name: 'Victor Glushkov', city: 'Rostov on Don', born: 1923 },
+  { name: 'Ibn Arabi', city: 'Murcia', born: 1165 },
+  { name: 'Mao Zedong', city: 'Shaoshan', born: 1893 },
+  { name: 'Rene Descartes', city: 'La Haye en Touraine', born: 1596 },
+];
+
+// Metadata
+
+const age = year =>
+  new Date().getFullYear() -
+  new Date(year + '').getFullYear();
+
+const upper = s => s.toUpperCase();
+
+const md = {
+  name: field('name'),
+  place: field('city', upper),
+  age: field('born', age)
+};
+
+// Usage
+
+const p1 = projection(md);
+const data = persons.map(p1);
+console.dir(data);
