What does ?: () => syntax mean?

Issue

I ran across this in an NPM module that I am using and I don't know what it means

getStorage?: () => StorageType

where getStorage is used:

export interface IAuthTokenInterceptorConfig {
  header?: string
  headerPrefix?: string
  requestRefresh: TokenRefreshRequest
  tokenExpireFudge?: number
  getStorage?: () => StorageType
}

Here is the definition of storage type:

export type StorageType = {
  remove(key: string): void
  set(key: string, value: string): void
  get(value: string): string | null
}

secondly... I think it is trying to tell me that I can provide my own storage provider.. but how do I do that?

Solution

getStorage is an optional property that if defined, must be a function that returns a StorageType, aka a factory.

For example, you could define a factory function to return a localStorage backed StorageType...

class LocalStorageType implements StorageType {
  remove(key: string) {
    localStorage.removeItem(key);
  }

  set(key: string, value: string) {
    localStorage.setItem(key, value);
  }

  get(key: string) {
    return localStorage.getItem(key);
  }
}

const myAuthTokenInterceptorConfig: IAuthTokenInterceptorConfig = {
  // other properties...
  getStorage: () => new LocalStorageType(),
}

Answered By - Phil