How to fix The return type does not match the base signature in TypeScript

// Base class with method signature
class Animal {
  makeSound(): string {
    return "generic animal sound";
  }
}

// Subclass with incompatible override
class Dog extends Animal {
  // ERROR: The return type does not match the base signature
  makeSound(): number {
    return 42; // returning number instead of string
  }
}

// Base interface
interface Shape {
  getArea(): number;
}

// Correct implementation - same return type
class Circle implements Shape {
  getArea(): number {
    return Math.PI * this.radius * this.radius;
  }
}

// Also correct - returning a subtype (number literal type)
class Square implements Shape {
  getArea(): 100 {  // 100 is assignable to number
    return 100;
  }
}

// ERROR - string not assignable to number
class Triangle implements Shape {
  getArea(): string {
    return "100";
  }
}

abstract class DataFetcher {
  abstract fetchData(): Promise<string>;
}

// Correct - same Promise type
class ApiFetcher extends DataFetcher {
  async fetchData(): Promise<string> {
    const response = await fetch("/api/data");
    return response.text();
  }
}

// ERROR - Promise<number> not assignable to Promise<string>
class NumberFetcher extends DataFetcher {
  async fetchData(): Promise<number> {
    return 42;
  }
}

// Also correct - returning a subtype
class JsonFetcher extends DataFetcher {
  async fetchData(): Promise<string> {
    const response = await fetch("/api/json");
    const data = await response.json();
    return JSON.stringify(data); // returns string
  }
}

class Vehicle {
  getEngine(): Engine {
    return new Engine();
  }
}

class Car extends Vehicle {
  // CORRECT - TurboEngine extends Engine, so this is covariant
  getEngine(): TurboEngine {
    return new TurboEngine();
  }
}

class Engine {}
class TurboEngine extends Engine {}

// This works because TurboEngine can be used wherever Engine is expected
const car = new Car();
const engine: Engine = car.getEngine(); // TurboEngine is assignable to Engine

interface Repository<T> {
  findById(id: string): T | null;
}

// Correct - same generic type
class UserRepository implements Repository<User> {
  findById(id: string): User | null {
    // implementation
    return null;
  }
}

// ERROR - return type doesn't match
class ProductRepository implements Repository<Product> {
  findById(id: string): string | null {  // string not assignable to Product
    return "product data";
  }
}

// For generic methods within classes
class Base {
  process<T>(value: T): T {
    return value;
  }
}

class Derived extends Base {
  // Can override with same signature
  process<T>(value: T): T {
    return super.process(value);
  }
  
  // ERROR - different return type
  process<T>(value: T): string {
    return "converted";
  }
}

class EventEmitter {
  // Base method returns void
  onEvent(): void {
    console.log("event");
  }
}

class MyEmitter extends EventEmitter {
  // CORRECT - can return a value even though base returns void
  onEvent(): string {
    console.log("my event");
    return "handled";  // This is allowed!
  }
}

class Reader {
  // Base method returns string
  read(): string {
    return "data";
  }
}

class MyReader extends Reader {
  // ERROR - void not assignable to string
  read(): void {
    console.log("reading");
  }
}

interface Calculator {
  calculate(x: number, y: number): number;
}

// Without explicit annotation, TypeScript might infer wrong type
class BadCalculator implements Calculator {
  calculate(x: number, y: number) {
    // TypeScript infers return type as string here
    return (x + y).toString(); // ERROR: string not assignable to number
  }
}

// With explicit annotation, error is clearer
class GoodCalculator implements Calculator {
  calculate(x: number, y: number): number {  // Explicit return type
    return x + y;  // Correct
  }
}

// Base with overloads
class Parser {
  parse(input: string): string;
  parse(input: number): number;
  parse(input: string | number): string | number {
    return input;
  }
}

// Override must handle all cases
class MyParser extends Parser {
  // ERROR - doesn't match all overloads
  parse(input: string): string {
    return input.toUpperCase();
  }
}

// Correct - matches all overloads
class CorrectParser extends Parser {
  parse(input: string): string;
  parse(input: number): number;
  parse(input: string | number): string | number {
    if (typeof input === "string") {
      return input.toUpperCase();
    } else {
      return input * 2;
    }
  }
}