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    Function keyIsIn

    • Type guard that checks if a key exists as an own property in an object.

      This function is similar to hasKey() but with reversed parameter order and different type narrowing behavior. While hasKey() narrows the object type, keyIsIn() narrows the key type to be a valid key of the given object.

      Type Narrowing Behavior:

      • Narrows the key type to be a key that exists in the object (K & keyof R)
      • Useful when you have a dynamic key and want to ensure it's valid for a specific object
      • The object type remains unchanged

      Implementation: Uses Object.hasOwn() to check for own properties (not inherited).

      Type Parameters

      • const K extends PropertyKey

        The type of the key to check, must extend PropertyKey (string | number | symbol)

      • const R extends UnknownRecord

        The type of the record (object), must extend UnknownRecord

      Parameters

      • key: K

        The key to check for

      • obj: R

        The object to check within

      Returns key is K & keyof R

      true if key is an own property of obj, false otherwise. When true, TypeScript narrows the key type to be a valid key of the object.

      const user = { id: 1, name: 'Ada' } as const;
      const maybeKey: string = 'name';

      if (keyIsIn(maybeKey, user)) {
        assert(user[maybeKey] === 'Ada');
      } else {
        assert.fail('Expected a known key.');
      }

      hasKey - Similar function that narrows the object type instead of the key type

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