Creating custom types in Typescript to avoid the "is not assignable to type..." error

Running into a challenge while using the graphql-request module from Prisma Labs. Specifically, I am unsure of how to define certain options using typescript.

In my request, there are 2 headers that need to be defined:

interface GraphQLHeaders {
  'X-Api-Key'?: string
  'X-Account-Id'?: string
}

However, when attempting to define these headers in TypeScript, I encounter errors:

 const headers: GraphQLHeaders = {}
 if(apiKey)
   headers['X-Api-Key'] = apiKey
 const client = new GraphQLClient(apiUrl, { headers })

The error message reads:

Type 'GraphQLHeaders' is not assignable to type 'Headers | string[][] | Record<string, string> | undefined'.
  Type 'GraphQLHeaders' is not assignable to type 'Record<string, string>'.
    Index signature is missing in type 'GraphQLHeaders'

I am seeking guidance on the correct approach for restricting the headers to certain fields while still being able to pass them successfully. Any suggestions?

typescript

Answer №1

One way to restrict keys in an object is by using a string index that allows only specified keys

interface GraphQLHeaders {
  'X-Api-Key'?: string
  'X-Account-Id'?: string
}

type WithIndex<T> = T & {
    [key: string]: never;
}

const headers: WithIndex<GraphQLHeaders> = {}

With the WithIndex utility type, you can still access known properties with their predefined value types. Accessing invalid keys will not trigger a TypeScript error, but their values will always be inferred as never/undefined.

const key: string | undefined = headers['X-Api-Key'];
const bad: undefined = headers['something'];

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