How to fix Property modifier cannot appear on a constructor declaration in TypeScript

// BEFORE: Error TS1248
class User {
  public constructor(name: string) { // Error: public on constructor
    // ...
  }
}

// AFTER: Remove the modifier
class User {
  constructor(name: string) { // OK: no modifier on constructor
    // ...
  }
}

// BEFORE: Trying to apply modifiers incorrectly
class User {
  constructor(public name: string, private age: number) { // Error: wrong syntax
    // ...
  }
}

// AFTER: Correct parameter property syntax
class User {
  constructor(
    public name: string,    // OK: creates public property this.name
    private age: number     // OK: creates private property this.age
  ) {
    // Properties are automatically created and initialized
  }
}

const user = new User("Alice", 30);
console.log(user.name); // OK: "Alice" (public)
console.log(user.age);  // Error: age is private

// BEFORE: Confusing syntax
class Product {
  constructor(private name: string, public price: number) {
    // ...
  }
}

// AFTER: Clear separation
class Product {
  private name: string;
  public price: number;

  constructor(name: string, price: number) {
    this.name = name;
    this.price = price;
  }
}

// Alternative with readonly
class Product {
  constructor(
    private readonly id: string, // OK: readonly parameter property
    public name: string
  ) {}
}

// BEFORE: Wrong singleton syntax
class Database {
  private static instance: Database;

  private constructor() { // Error if used incorrectly
    // ...
  }

  static getInstance(): Database {
    if (!Database.instance) {
      Database.instance = new Database();
    }
    return Database.instance;
  }
}

// AFTER: Correct private constructor
class Database {
  private static instance: Database;

  private constructor() { // OK: private constructor (no parameters)
    // Initialization code
  }

  static getInstance(): Database {
    if (!Database.instance) {
      Database.instance = new Database();
    }
    return Database.instance;
  }
}

// Usage
const db = Database.getInstance(); // OK
const db2 = new Database(); // Error: constructor is private

// BEFORE: Method syntax on constructor
class Calculator {
  public constructor(public value: number = 0) { // Error: public on constructor
    // ...
  }

  public add(x: number): void {
    this.value += x;
  }
}

// AFTER: Correct syntax
class Calculator {
  constructor(public value: number = 0) { // OK: parameter property with default
    // ...
  }

  public add(x: number): void {
    this.value += x;
  }
}

// Common confusion: constructor vs method
class Example {
  // Constructor - no return type, named "constructor"
  constructor(public x: number) {}

  // Method - has return type, custom name
  public method(y: number): void {
    console.log(this.x + y);
  }
}

// tsconfig.json - ensure proper settings
{
  "compilerOptions": {
    "target": "ES2022", // or later
    "lib": ["ES2022", "DOM"],
    "module": "commonjs",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "experimentalDecorators": false, // Unless you need decorators
    "emitDecoratorMetadata": false
  }
}

# Check TypeScript version
npx tsc --version

# Update if needed
npm install typescript@latest --save-dev