Simple crawler

crawl.js

// https://stackoverflow.com/a/75251817

// crawl :: ((a -> Promise b), (b -> [a]), (b -> c), ((Map a c) -> d), Int?) -> [a] -> Promise d
const crawl = (search, extract, convert, collect, simul = 1) => async (inits) => {
  const toCheck = new Set (inits)
  const found = new Map ()
  while (toCheck .size !== 0) {
    const nextBlock = [...toCheck .values ()] .slice (0, simul)
    const results = await Promise .all (nextBlock .map ((x) => search (x)))
    results .forEach ((res, i) => {
      const key = nextBlock [i]
      found .set (key, convert (res))
      const newVals = extract (res)
      newVals .forEach ((val) => found .has (val) || toCheck .add (val))
      toCheck .delete (key)
    })
  } 
  return collect (found)
}

// Exmaple code

const get = ((data) => (id) => console .log (`Getting ${id}`) || 
  new Promise ((res, rej) => setTimeout (() => res (data [id]), 1000))
)({  // don't look for any structure here.  The fruits were more fun than "a", "b", "c", ...
  1: {val: ['Apples', 'Apricots'], kids: [3, 5]},
  2: {val: ['Bananas', 'Boysenberries', 'Blackberries', 'Blueberries'], kids: [3, 4]},
  3: {val: ['Cherries', 'Cantaloupes', 'Clementines'], kids: []},
  4: {val: ['Dates', 'Dewberries', 'Dragon Fruit'], kids: [1, 4, 5]},
  5: {val: ['Elderberries', 'Eggfruit'], kids: [2]},
  6: {val: ['Fig', 'Farkleberries'], kids: [6, 7]},
  7: {val: ['Grapefruit', 'Grapes', 'Gooseberries', 'Guava'], kids: []},
}) 

const findFruit = crawl (
  get,
  (x) => x .kids, 
  (x) => x .val, 
  (m) => [... m .values ()],
  3
)

findFruit ([1]) // or, say, [1, 6]
  .then (letters => console .log ('Results:', letters))
  .catch (err => console .log (`Error: ${err}`))