Use .on() to subscribe

Author: tshemsedinov

JavaScript/6-on.js

@@ -0,0 +1,83 @@
+'use strict';
+
+const { EventEmitter } = require('events');
+
+const DEFERRED_PENDING = 0;
+const DEFERRED_RESOLVED = 1;
+const DEFERRED_REJECTED = 2;
+
+class Deferred extends EventEmitter {
+  constructor(onDone = null, onFail = null) {
+    super();
+    this.value = undefined;
+    if (onDone) this.on('done', onDone);
+    if (onFail) this.on('fail', onFail);
+    this.status = DEFERRED_PENDING;
+  }
+
+  isPending() {
+    return this.status === DEFERRED_PENDING;
+  }
+
+  isResolved() {
+    return this.status === DEFERRED_RESOLVED;
+  }
+
+  isRejected() {
+    return this.status === DEFERRED_REJECTED;
+  }
+
+  done(callback) {
+    this.on('done', callback);
+    if (this.isResolved()) callback(this.value);
+    return this;
+  }
+
+  fail(callback) {
+    this.on('fail', callback);
+    if (this.isRejected()) callback(this.value);
+    return this;
+  }
+
+  resolve(value) {
+    this.value = value;
+    this.status = DEFERRED_RESOLVED;
+    this.emit('done', value);
+    return this;
+  }
+
+  reject(value) {
+    this.value = value;
+    this.status = DEFERRED_REJECTED;
+    this.emit('fail', value);
+    return this;
+  }
+}
+
+// Usage
+
+const persons = {
+  10: 'Marcus Aurelius',
+  11: 'Mao Zedong',
+  12: 'Rene Descartes',
+};
+
+const getPerson = id => {
+  const result = new Deferred();
+  setTimeout(() => {
+    const name = persons[id];
+    if (name) result.resolve({ id, name });
+    else result.reject(new Error('Person is not found'));
+  }, 1000);
+  return result;
+};
+
+const d1 = getPerson(10);
+d1.on('done', value => console.log('Resolved d1', value));
+d1.on('fail', error => console.log('Resolved d1', error));
+console.dir({ d1 });
+
+const d2 = getPerson(20);
+d2.on('done', value => console.log('Resolved d2', value));
+d2.on('fail', error => console.log('Resolved d2', error.message));
+console.dir({ d2 });