Add state pattern example for solution

Author: leemhoon00

src/State/Book/index.ts

@@ -0,0 +1,70 @@
+export interface State {
+  render(): void;
+  publish(): void;
+}
+
+class Document {
+  private state: State;
+
+  constructor() {
+    this.state = new DraftState(this);
+  }
+
+  changeState(state: State) {
+    this.state = state;
+  }
+
+  render() {
+    this.state.render();
+  }
+
+  publish() {
+    this.state.publish();
+  }
+}
+
+class DraftState implements State {
+  constructor(private document: Document) {}
+
+  render() {
+    console.log("Rendering the document in Draft state");
+  }
+
+  publish() {
+    console.log("Moving the document to Moderation state");
+    this.document.changeState(new ModerationState(this.document));
+  }
+}
+
+class ModerationState implements State {
+  constructor(private document: Document) {}
+
+  render() {
+    console.log("Rendering the document in Moderation state");
+  }
+
+  publish() {
+    console.log("Making the document public in Published state");
+    this.document.changeState(new PublishedState(this.document));
+  }
+}
+
+class PublishedState implements State {
+  constructor(private document: Document) {}
+
+  render() {
+    console.log("Rendering the document in Published state");
+  }
+
+  publish() {
+    console.log("The document is already in Published state. Nothing to do.");
+  }
+}
+
+const document = new Document();
+
+document.render();
+document.publish();
+document.publish();
+document.render();
+document.publish();