Typescript call by need aka lazy

lazy.ts

export type Lazy<Value> = () => Value

export function lazy<Value>(thunk: () => Value){
  let value: Value // will hold computed value
  let factory: (() => Value) | null = thunk // used as check if value is computed and allow thunk to be garbage collected
  return () => {
    if (!factory) {
      return value
    } else {
      value = factory()
      factory = null
      return value
    }
  }
}