I encountered an issue where TypeScript does not recognize the type of a variable resulting from object destructuring

I'm attempting to utilize a variable from a destructuring expression as part of another object, but Typescript is not correctly recognizing its type.

Here is an example of what I am trying to achieve:

// defining a data structure
type Data = {
  firstName: string;
  lastName: string;
  email: string;
  tags: string[];
  documents: [{ name: string; content: string }];
};

// generating dummy data
const getData = (): Data => ({
  firstName: 'John',
  lastName: 'Doe',
  email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="573d383f397933383217322f363a273b327934383a">[email protected]</a>',
  tags: ['javascript', 'typescript'],
  documents: [{ name: 'cv.pdf', content: '...' }],
});

const processData = (data: {
  // why is this type not capturing metadata from the rest operator?
  [key: string]:
    | string
    | string[]
    | { [key: string]: string }[]
    | { [key: string]: string };
}): void => {
  console.log(data);
};

// destructuring with rest
const { email, tags, documents, ...meta } = getData();

// why is Typescript not recognizing the type here?
processData({ email, tags, documents, meta });

and here is the error message that I am encountering

 error TS2322: Type '{ firstName: string; lastName: string; }' is not assignable to type 'string | string[] | { [key: string]: string; } | { [key: string]: string; }[]'.
  Type '{ firstName: string; lastName: string; }' is not assignable to type '{ [key: string]: string; }'.
    Index signature is missing in type '{ firstName: string; lastName: string; }'.

50 processData({ email, tags, documents, meta });
                                         ~~~~

  src/example.ts:37:3
     37   [key: string]:
          ~~~~~~~~~~~~~~
     38     | string
        ~~~~~~~~~~~~
    ... 
     40     | { [key: string]: string }[]
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     41     | { [key: string]: string };
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The expected type comes from this index signature.

Can anyone point out what I might be doing incorrectly?

Answer №1

In TypeScript, types are not completely rigid. They specify required fields but do not prevent additional properties from being added. This means that it is possible to insert properties into the meta object that may not align with the expected index signature in the processData function (for example, using

Object.assign(meta, { id: 4234 })
).

To resolve this issue, one solution is to destructure the meta object into a new one.

processData({ email, tags, documents, {...meta} });

Alternatively, you can define constraints for additional properties in the Data type:

type Data = {
  firstName: string;
  lastName: string;
  email: string;
  tags: string[];
  documents: [{ name: string; content: string }];
  [key: string]: string | string[] | {name: string, content: string}[];
}; 

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