A typed LocalStorage interface - with two versions

typedLocalStorage.ts

// With a normal object type
type LocalStorage = {
  user: {
    email: string
    age: number
  }
  company: {
    name: string
  }
}

const typedLocalStorage = {
  setItem: <T extends keyof LocalStorage>(key: T, value: LocalStorage[T]) => {
    localStorage.setItem(key, value ? JSON.stringify(value) : undefined)
  },

  getItem: <T extends keyof LocalStorage>(key: T): LocalStorage[T] | undefined => {
    const value = localStorage.getItem(key)

    if (value) {
      return JSON.parse(value) as LocalStorage[T]
    } else {
      return undefined
    }
  }
}

typedLocalStorage.setItem('user', { email: 'a', age: 2 })
const user: LocalStorage['user'] | undefined = typedLocalStorage.getItem('user')

typedLocalStorage_discriminated_union.ts

// With a discriminated union type
type LocalStorage =
  | { key: 'user', value: { email: string, age: number } }
  | { key: 'company', value: { name: string } }

const typedLocalStorage = {
  setItem: <T extends LocalStorage['key']>(key: T, value: Extract<LocalStorage, { key: T, value: unknown }>['value']) => {
    localStorage.setItem(key, value ? JSON.stringify(value) : undefined)
  },

  getItem: <T extends LocalStorage['key']>(key: T): Extract<LocalStorage, { key: T, value: unknown }>['value'] | undefined => {
    const value = localStorage.getItem(key)

    if (value)
      return JSON.parse(value)
    else
      return undefined
  }
}

typedLocalStorage.setItem("company", { name: 'AB' })
const company: { name: string } | undefined = typedLocalStorage.getItem("company")