Get the current value of subscribable stores, e.g. Svelte stores

get.ts

/** Callback to inform of a value updates. */
type Subscriber<T> = (value: T) => void

/** Unsubscribes from value updates. */
type Unsubscriber = () => void

/** A store that can be subscribed */
interface Subscribable<T> {
  subscribe: (run: Subscriber<T>) => Unsubscriber
}

// Solution 1: WeakMap

const storeMap = new WeakMap()

/** Get the store's current value */
export function get<T>(store: Subscribable<T>): T {
  if (!storeMap.has(store)) {
    store.subscribe((v) => storeMap.set(store, v))
  }

  return storeMap.get(store)
}

// Solution 2: Manual subscription

/** Utility to auto-subscribe store values */
export function useGet<T>(store: Subscribable<T>): [() => T, Unsubscriber] {
  let value: T
  const unsub = store.subscribe((v) => (value = v))
  return [() => value, unsub]
}

// Solution 3: Sub and unsub (Svelte's built-in function)

/** Subscribes and unsubscrines immediately to get the current value */
export function quickGet<T>(store: Subscribable<T>): T {
  let value: T
  store.subscribe((v) => (value = v))()
  return value
}