Is it not possible to type a Specific Object Type as a Record?

Question

Is it not possible to type a Specific Object Type as a Record?

I am looking to create a generic Table Row interface that will only allow objects with primitive attribute values. However, when I try to assign a specific type of object to the row, it fails. Why is this happening and how can I make it work? My goal is to restrict rows to objects with primitive attribute values.

Link to Playground

// Defining a Table Row
export type Row = Record<string, string | number | undefined>

// Example User Interface
interface User { name: string }
const jim: User = { name: 'Jim' }
const row: Row = jim // <== Error

Error Message:

Type 'User' is not assignable to type 'Row'.
  Index signature for type 'string' is missing in type 'User'.

typescript

Answer 1

Answer №1

When extending interfaces, it is important to consider the limitations imposed by index types. An object that extends User and is stored in a User-typed variable could potentially have additional properties beyond what was originally defined.

// User
interface User { name: string }
interface FooUser extends User { foo(): void; }
const jim: FooUser = { name: 'Jim', foo() {} }
const jimAsUser: User = jim;
const row: Row = jim;  // this will fail as expected:
                       // foo is not one of the specified property types!

It's worth noting that using type instead of interface provides a safer approach, as extra properties in subinterfaces are not anticipated. Nevertheless, TypeScript does allow for this behavior, even though it may not be ideal. The differences between type and interface are subtle, as mentioned in the TypeScript handbook (though without going into specific cases).

// UserAsType
type UserAsType = { name: string };
const jimAsType: UserAsType = { name: 'Jim' }
const jimAsTypeFromFooUser: UserAsType = jim;  // this should ideally fail
const row2: Row = jimAsType

Playground Link

Answer 2

When extending interfaces, it is important to consider the limitations imposed by index types. An object that extends User and is stored in a User-typed variable could potentially have additional properties beyond what was originally defined.

// User
interface User { name: string }
interface FooUser extends User { foo(): void; }
const jim: FooUser = { name: 'Jim', foo() {} }
const jimAsUser: User = jim;
const row: Row = jim;  // this will fail as expected:
                       // foo is not one of the specified property types!

It's worth noting that using type instead of interface provides a safer approach, as extra properties in subinterfaces are not anticipated. Nevertheless, TypeScript does allow for this behavior, even though it may not be ideal. The differences between type and interface are subtle, as mentioned in the TypeScript handbook (though without going into specific cases).

// UserAsType
type UserAsType = { name: string };
const jimAsType: UserAsType = { name: 'Jim' }
const jimAsTypeFromFooUser: UserAsType = jim;  // this should ideally fail
const row2: Row = jimAsType

Playground Link

Is it not possible to type a Specific Object Type as a Record?

Answer №1

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