How to fix Method 'X' in abstract class must be abstract or have implementation in TypeScript

Find the method that triggers the error. It will look like a method signature without a body and without the abstract keyword. The error message tells you the exact method name and class.

You have two options:

Option 1: Add a method body (implementation)

abstract class Animal {
  // WRONG - missing body and abstract keyword
  makeSound();
  
  // CORRECT - has implementation
  makeSound() {
    console.log("Generic animal sound");
  }
}

Option 2: Mark the method as abstract

abstract class Animal {
  // WRONG - missing abstract keyword and body
  makeSound();
  
  // CORRECT - marked abstract, no body needed
  abstract makeSound(): void;
}

Choose Option 1 if the method should have default behavior that all subclasses can use. Choose Option 2 if each subclass must provide its own implementation.

When you mark a method as abstract, every non-abstract class that extends your abstract class must implement that method. If you forget, TypeScript will show an error on the subclass.

abstract class Animal {
  abstract makeSound(): void;
}

// WRONG - does not implement makeSound()
class Dog extends Animal {
  // Error: Missing implementation of abstract method 'makeSound'
}

// CORRECT - implements the abstract method
class Dog extends Animal {
  makeSound(): void {
    console.log("Woof!");
  }
}

Ensure your method declaration includes the return type. If the method returns void, explicitly state it. This makes your abstract contract clear to implementers.

abstract class Calculator {
  // Good - return type is clear
  abstract add(a: number, b: number): number;
  abstract log(message: string): void;
  
  // Bad - missing return type
  abstract multiply(a, b);
}

Make sure the parent class itself is marked with the abstract keyword. Without it, you cannot declare abstract methods at all.

// WRONG - class must be abstract
class Animal {
  abstract makeSound(): void; // Error: only abstract classes can have abstract methods
}

// CORRECT - class is marked abstract
abstract class Animal {
  abstract makeSound(): void;
}

Most editors like VS Code offer a quick fix when you hover over the error. Click "Implement inherited abstract members" or similar to auto-generate method stubs in subclasses. This saves time and prevents mistakes.

After fixing the issue, run tsc --noEmit or npm run build to verify the error is gone. Make sure all subclasses properly implement abstract methods, and test your code to ensure it behaves as expected.