54. flatten Thunk

Suppose we have a Callback type

type Callback = 
  (error: Error, result: any | Thunk) => void

A Thunk is a function that take a Callback as parameter

type Thunk = (callback: Callback) => void

Like following three thunks

const func1 = (cb) => {
  setTimeout(() => cb(null, 'ok'), 10)
}

const func2 = (cb) => {
  setTimeout(() => cb(null, func1), 10)
}

const func3 = (cb) => {
  setTimeout(() => cb(null, func2), 10)
}

in above example, three functions are kind of chained up, func3 → func2 → func1, but it don't work without some glue.

OK, now you are asked to implement a flattenThunk() which glue them up and returns a new thunk.

flattenThunk(func3)((error, data) => {
   console.log(data) // 'ok'
})

note

Once error occurs, the rest uncalled functions should be skipped