feat: add private fields and default parameters to classes, update README and tests

Author: kentcdodds

exercises/01.classes/02.problem.constructors/README.mdx

@@ -0,0 +1,57 @@
+# Private Fields and Defaults
+
+👨‍💼 Classes can hide implementation details using private fields and provide
+flexibility with default parameter values.
+
+## Private Fields
+
+JavaScript has native private fields using the `#` prefix. These fields are
+truly private—they can only be accessed inside the class:
+
+```ts
+class BankAccount {
+	#balance: number = 0 // Truly private - not accessible outside the class
+
+	deposit(amount: number) {
+		this.#balance += amount
+	}
+}
+
+const account = new BankAccount()
+account.#balance // ❌ Error! Private field not accessible
+```
+
+Private fields enable **encapsulation**—hiding internal implementation details
+from code outside the class. This prevents accidental modification and makes
+your code more maintainable.
+
+## Default Parameter Values
+
+Constructors can use default parameter values to make some parameters optional:
+
+```ts
+class User {
+	name: string
+	role: string
+
+	constructor(name: string, role: string = 'user') {
+		this.name = name
+		this.role = role
+	}
+}
+
+const user = new User('Alice') // role defaults to 'user'
+const admin = new User('Bob', 'admin') // role explicitly set
+```
+
+🐨 Open <InlineFile file="index.ts" /> and:
+
+1. Use `#` prefix to create private fields
+2. Use default parameter values in constructors
+3. Create classes that encapsulate their internal state
+
+💰 Use `#` to declare a private field—it's truly private, not just a convention.
+
+💰 Default parameters let you create flexible constructors with sensible defaults.
+
+📜 [MDN - Private class features](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields)

exercises/01.classes/02.problem.private-fields-and-defaults/README.mdx

@@ -1,5 +1,4 @@
-// Constructor Patterns
-// 🦉 This runtime does not support parameter property shorthand.
+// Private Fields and Defaults
 
 // 🐨 Create a User class with these fields:
 // - name: string

…classes/02.problem.constructors/index.ts …lem.private-fields-and-defaults/index.tsexercises/01.classes/02.problem.constructors/index.ts renamed to exercises/01.classes/02.problem.private-fields-and-defaults/index.ts exercises/01.classes/02.problem.constructors/index.ts renamed to exercises/01.classes/02.problem.private-fields-and-defaults/index.ts

@@ -1,5 +1,5 @@
 {
-  "name": "exercises_01.classes_02.solution.constructors",
+  "name": "exercises_01.classes_02.problem.private-fields-and-defaults",
   "type": "module",
   "scripts": {
     "start": "npx @kentcdodds/log-module@latest ./index.ts",

…es/02.solution.constructors/package.json …private-fields-and-defaults/package.jsonexercises/01.classes/02.solution.constructors/package.json renamed to exercises/01.classes/02.problem.private-fields-and-defaults/package.json exercises/01.classes/02.solution.constructors/package.json renamed to exercises/01.classes/02.problem.private-fields-and-defaults/package.json

@@ -0,0 +1,7 @@
+# Private Fields and Defaults
+
+👨‍💼 Great work using private fields and default values!
+
+🦉 Private fields (`#`) provide true encapsulation—they're not accessible
+outside the class, preventing accidental modification of internal state. Default
+parameter values make your constructors flexible while keeping the code clean.

exercises/01.classes/02.solution.constructors/README.mdx

@@ -1,4 +1,4 @@
-// Constructor Patterns
+// Private Fields and Defaults
 
 class User {
 	name: string

exercises/01.classes/02.solution.private-fields-and-defaults/README.mdx

@@ -1,5 +1,5 @@
 {
-  "name": "exercises_01.classes_02.problem.constructors",
+  "name": "exercises_01.classes_02.solution.private-fields-and-defaults",
   "type": "module",
   "scripts": {
     "start": "npx @kentcdodds/log-module@latest ./index.ts",

…s/02.solution.constructors/index.test.ts …rivate-fields-and-defaults/index.test.tsexercises/01.classes/02.solution.constructors/index.test.ts renamed to exercises/01.classes/02.solution.private-fields-and-defaults/index.test.ts exercises/01.classes/02.solution.constructors/index.test.ts renamed to exercises/01.classes/02.solution.private-fields-and-defaults/index.test.ts

@@ -7,7 +7,8 @@ You learned:
 - 📦 **Fields** store data on instances
 - 🔧 **Methods** define behavior
 - 🏗️ **Constructors** initialize new instances
-- ⚡ **Parameter properties** as a shorthand
+- 🔒 **Private fields** (`#`) encapsulate internal state
+- ⚙️ **Default parameters** make constructors flexible
 
 🦉 Classes vs plain objects: Use classes when you need: