A getter that calls generator() to create a new instance and subsequently returns the same instance while it isValid()

valid-getter.ts

export function makeValidGetter<T extends {isValid(): boolean}>(generator: () => Promise<T>): () => Promise<T> {
  let value: T|undefined
  let promise: Promise<T>|undefined
  return async function() {
    if (value?.isValid()) return value
    if (promise) return await promise
    promise = generator()
      .then(result => {
        value = result
        promise = undefined
        return value
      })
    return await promise
  }
}