diff --git a/dist/app.js b/dist/app.js
index dbda66a..e69de29 100644
--- a/dist/app.js
+++ b/dist/app.js
@@ -1,29 +0,0 @@
-"use strict";
-var Department = /** @class */ (function () {
-    function Department(id, name) {
-        this.id = id;
-        this.name = name;
-        // private id: string;
-        // private name: string;
-        this.employees = [];
-        // this.id = id;
-        // this.name = n
-    }
-    Department.prototype.describe = function () {
-        console.log("Department (".concat(this.id, "): ").concat(this.name));
-    };
-    Department.prototype.addEmployee = function (employee) {
-        this.employees.push(employee);
-    };
-    Department.prototype.printEmployeeInformation = function () {
-        console.log(this.employees.length);
-        console.log(this.employees);
-    };
-    return Department;
-}());
-var accounting = new Department('1', 'Accounting');
-accounting.addEmployee('Max');
-accounting.addEmployee('Manu');
-// accounting.employees[2] = 'Anna';
-accounting.describe();
-accounting.printEmployeeInformation();
diff --git a/src/app.ts b/src/app.ts
index 158684d..4ab88f1 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -2,8 +2,8 @@ class Department {
   // private id: string;
   // private name: string;
   private employees: string[] = [];
-
-  constructor(private id: string, public name: string) {
+  // readonly는 프로퍼티를 초기화한 후 수정할 수 없다. 즉, 한번 할당 되면 변경되면 안되는 고유 번호들을 설정할 때 readonly를 사용한다.
+  constructor(private readonly id: string, public name: string) {
     // this.id = id;
     // this.name = n
   }
@@ -12,6 +12,7 @@ class Department {
   }
 
   addEmployee(employee: string) {
+    // this.id = '2';  // readonly이기 때문에 error가 발생한다.
     this.employees.push(employee);
   }
   printEmployeeInformation() {