What is the reason for TypeScript not throwing an error when an interface is not implemented correctly?

Question

What is the reason for TypeScript not throwing an error when an interface is not implemented correctly?

In my current scenario, I have a class that implements an interface. Surprisingly, the TypeScript compiler does not throw an error if the class fails to include the required method specified by the interface; instead, it executes with an error.

Is there a way to ensure that the compiler throws an error when a required method is missing?


interface PersonInterface {
  name: string;  
  age: number;  
}
class Person implements PersonInterface {    
  name: string = 'Mary';  
  foo: any = 'abc';
}

alert(new Person().name)

You can view and test the code here at the playground here

View my code on CodeSandbox: here

typescript typescript-typings typescript-generics

Answer 1

Answer №1

There seems to be an issue with the code in the playground when it comes to the age property, but here is how you can define a method:

Defining a Method

interface PersonInterface {
  name: string;  
  age: number;

  // Define a method
  sayHello(): string

  // Define a function property
  repeat: (sentence: string) => string
}

class Person implements PersonInterface {    
  name: string = 'Mary';  
  foo: any = 'abc';

  sayHello() {
    return `Hello my name is ${this.name}`
  }

  repeat = (input) => {
    return `${input}`
  }
}

If you want to deliberately cause an error, consider using an abstract class. Keep in mind that you cannot instantiate an abstract class. Documentation

abstract class PERSON {
  abstract name: string;
  abstract age: number;
  abstract repeat(sentence: string): string;
}

class Person extends PERSON{
  //
  // Will produce errors for not implementing
  // properties and methods
  //
}

To resolve the errors:

class Person extends PERSON {
  constructor(public name: string, public age: number) {
    super();
  }

  repeat(input: string) {
    return `${input}`;
  }
}

Answer 2

There seems to be an issue with the code in the playground when it comes to the age property, but here is how you can define a method:

Defining a Method

interface PersonInterface {
  name: string;  
  age: number;

  // Define a method
  sayHello(): string

  // Define a function property
  repeat: (sentence: string) => string
}

class Person implements PersonInterface {    
  name: string = 'Mary';  
  foo: any = 'abc';

  sayHello() {
    return `Hello my name is ${this.name}`
  }

  repeat = (input) => {
    return `${input}`
  }
}

If you want to deliberately cause an error, consider using an abstract class. Keep in mind that you cannot instantiate an abstract class. Documentation

abstract class PERSON {
  abstract name: string;
  abstract age: number;
  abstract repeat(sentence: string): string;
}

class Person extends PERSON{
  //
  // Will produce errors for not implementing
  // properties and methods
  //
}

To resolve the errors:

class Person extends PERSON {
  constructor(public name: string, public age: number) {
    super();
  }

  repeat(input: string) {
    return `${input}`;
  }
}

What is the reason for TypeScript not throwing an error when an interface is not implemented correctly?

Answer №1

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