Does Typescript fail to recognize the "delete" operator?

Question

Does Typescript fail to recognize the "delete" operator?

Whenever I utilize the delete operator in Typescript, it appears that the system does not recognize that the property has been eliminated. For instance:

interface HasName {
  name: string;
}

interface HasNoName {
  name: never;
}

function removeName(
  input: HasName,
): HasNoName {
  const output = {...input};
  delete output.name;
  return output; // This results in an error
}

Typescript disapproves of the function with the following error message:

 Type '{ name: string; }' is not compatible with type 'HasNoName'.
  Properties of 'name' are not matching.
    String cannot be assigned to 'never'.

In my view, upon removing the name property, Typescript should interpret output as being compliant with HasNoName, but this does not seem to be the case.

I can easily find a workaround, such as

const nameless = output as HasNoName; return nameless

. However, I am curious to understand WHY this error occurs and whether it is intentional or a bug?

javascript typescript

Answer 1

Answer №1

The issue lies not in the delete operator itself, but in the fact that the return statement does not automatically "Cast" the output to the correct type if it doesn't match (for example, when hasName is not equal to HasNoName). In such cases, you will need to perform an explicit cast and take on the associated risks. Here's how you can do it:

removeName(
    input: HasName,
  ): HasNoName {
    const output = {...input};
    delete output.name;
    return <HasNoName>output;
  }

I trust this explanation proves useful to you.

Answer 2

The issue lies not in the delete operator itself, but in the fact that the return statement does not automatically "Cast" the output to the correct type if it doesn't match (for example, when hasName is not equal to HasNoName). In such cases, you will need to perform an explicit cast and take on the associated risks. Here's how you can do it:

removeName(
    input: HasName,
  ): HasNoName {
    const output = {...input};
    delete output.name;
    return <HasNoName>output;
  }

I trust this explanation proves useful to you.

Answer 3

Answer №2

It is recommended to steer clear of using the delete operator. Instead, opt for object destructuring:

interface Person {
  firstName: string;
  lastName: string;
}

function removeLastName(
  person: Person,
): Omit<Person, 'lastName'> {
  const { lastName: _, ...updatedPerson } = person;
  return updatedPerson;
}

Answer 4

It is recommended to steer clear of using the delete operator. Instead, opt for object destructuring:

interface Person {
  firstName: string;
  lastName: string;
}

function removeLastName(
  person: Person,
): Omit<Person, 'lastName'> {
  const { lastName: _, ...updatedPerson } = person;
  return updatedPerson;
}

Does Typescript fail to recognize the "delete" operator?

Answer №1

Answer №2

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