@@ -5,6 +5,10 @@
     constructor(subject) {
       subject.subscribe(this);
     }
+
+    update() {
+      throw new Error('Observer: the `update` method should be overridden.');
+    }
   }
 
   window.Observer = Observer;
 
@@ -7,9 +7,6 @@
     }
 
     subscribe(observer = {}) {
-      if (!(typeof observer.update === 'function')) {
-        throw new Error(`Observer must implement an 'update' method.`);
-      }
       this.observers.add(observer);
       return () => this.observers.delete(observer);
     }
 
@@ -5,5 +5,9 @@
     subject.subscribe(this);
   }
 
+  Observer.prototype.update = function() {
+    throw new Error('Observer: the `update` method should be overridden.');
+  };
+
   window.Observer = Observer;
 }
@@ -6,9 +6,6 @@
   }
 
   Subject.prototype.subscribe = function(observer = {}) {
-    if (!(typeof observer.update === 'function')) {
-      throw new Error(`Observer must implement an 'update' method.`);
-    }
     this.observers.add(observer);
     return () => this.observers.delete(observer);
   };
Original file line number	Diff line number	Diff line change
`@@ -5,6 +5,10 @@`
`5`	`5`	`constructor(subject) {`
`6`	`6`	`subject.subscribe(this);`
`7`	`7`	`}`
	`8`	`+`
	`9`	`+ update() {`
	`10`	+ throw new Error('Observer: the `update` method should be overridden.');
	`11`	`+ }`
`8`	`12`	`}`
`9`	`13`
`10`	`14`	`window.Observer = Observer;`
Original file line number	Diff line number	Diff line change
`@@ -7,9 +7,6 @@`
`7`	`7`	`}`
`8`	`8`
`9`	`9`	`subscribe(observer = {}) {`
`10`		`- if (!(typeof observer.update === 'function')) {`
`11`		- throw new Error(`Observer must implement an 'update' method.`);
`12`		`- }`
`13`	`10`	`this.observers.add(observer);`
`14`	`11`	`return () => this.observers.delete(observer);`
`15`	`12`	`}`
Original file line number	Diff line number	Diff line change
`@@ -5,5 +5,9 @@`
`5`	`5`	`subject.subscribe(this);`
`6`	`6`	`}`
`7`	`7`
	`8`	`+ Observer.prototype.update = function() {`
	`9`	+ throw new Error('Observer: the `update` method should be overridden.');
	`10`	`+ };`
	`11`	`+`
`8`	`12`	`window.Observer = Observer;`
`9`	`13`	`}`
Original file line number	Diff line number	Diff line change
`@@ -6,9 +6,6 @@`
`6`	`6`	`}`
`7`	`7`
`8`	`8`	`Subject.prototype.subscribe = function(observer = {}) {`
`9`		`- if (!(typeof observer.update === 'function')) {`
`10`		- throw new Error(`Observer must implement an 'update' method.`);
`11`		`- }`
`12`	`9`	`this.observers.add(observer);`
`13`	`10`	`return () => this.observers.delete(observer);`
`14`	`11`	`};`