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

Similar questions

The system has encountered an issue: "EntityMetadataNotFound: Unable to locate metadata for the entity named 'User

Building a versatile and interactive table using AngularJS with data sourced from a

Tips for Implementing a "Please Hold On" Progress Bar in ASP.NET

Using Angular2, you can dynamically assign values to data-* attributes

Expressjs - Error: Headers already sent to the client and cannot be set

Should the Express.js router be required directly or stored in a variable before use?

Can new content be incorporated into an existing JSON file using HTML input and the fs.writeFile function?

The npm script for running Protractor is encountering an error

Merge two arrays of objects containing Google Map coordinates

The type '{ }' does not include the properties 'params', 'isExact', 'path', 'url' from the 'match<Identifiable>' type

Node-Fetch encounters problematic JSON response with Danish characters

issue with for loop in jquery ajax not processing complete response data

Triggering the function once the text field is tapped again

determine the values of objects based on their corresponding keys

How to prevent collapse when selecting a node in React.js Mui Treeview

Tips for creating a cookie for an alternate website upon launching a new browser tab

Tips for ensuring that the "this" in React ES6 static method is bound to the lexical scope

Using JavaScript to call a PHP file as the source file

What is causing this setInterval function to run repeatedly?

Using AngularJS to bind objects with a checkbox