The interface does not allow properties to be assigned as string indexes

Below are the interfaces I am currently working with:

export interface Meta {
  counter: number;
  limit: number;
  offset: number;
  total: number;
}

export interface Api<T> {
  [key: string]: T[];
  meta: Meta; // encountered an error here
}

I have encountered the following error message:

The property 'meta' of type 'Meta' is not assignable to the string index type 'T[]'.

Upon further investigation, I came across this information in the TypeScript documentation:

String index signatures allow for describing a "dictionary" pattern but they require all properties to match their return types. This is because a string index declaration implies that obj.property is also accessed as obj["property"].

Does this mean that when using a string index signature, all variables must match this specific type?

To resolve the error, I can modify the interface declaration as follows:

export interface Api<T> {
  [key: string]: any; // used 'any' here as a workaround
  meta: Meta;
}

However, this approach eliminates the ability for complete type inference. Is there a more elegant solution to address this issue?

Answer №1

If you want to combine two interfaces, you can create an intersection of them:

interface UserData<T> {
    [key: string]: T[];  
}

type UserDataType<T> = UserData<T> & {
    info: Info;
}

declare let user: UserDataType<string>;

let userData = user.info // variable `userData` now has type `Info`
let moreData = user["info"]; // variable `moreData` now has type `Info`

let additionalDataOne = user["someotherindex"] // type of `additionalDataOne` is `string[]`
let additionalDataTwo = user.someotherindex // type of `additionalDataTwo` is `string[]`

Answer №2

When attempting to utilize the recommended solution, I encountered issues with its functionality within my implementation. As a workaround, I resorted to nesting a portion of the code with a dynamic key. Here is the modified approach:

interface MultichannelConfiguration {
  channels: {
    [key: string]: Configuration;
  }
  defaultChannel: string;
}

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