Using graphql/dataloader as a reference, this is an experiment WRT how to implement code that does the same thing, with the same API

dataloader-batching-experiment.js

function DataLoader (_batchLoadingFn) {
  this._timer
  this._keys = []
  this._batchLoadingFn = _batchLoadingFn
}

DataLoader.prototype.load = function(key) {
  clearTimeout(this._timer)
  const promisedValue = new Promise ( resolve => this._keys.push({key, resolve}) )

  this._timer = setTimeout(() => {
    this._batchLoadingFn(this._keys.map(k => k.key))
      .then(values => {
        this._keys.forEach(({resolve}, i) => {
          resolve(values[i])
        })
        this._keys = []
      })
  }, 0)

  return promisedValue
}


const batchLoadingFunction = keys => new Promise( resolve => resolve(keys.map(k => k * 2)) )
const loader = new DataLoader(batchLoadingFunction)

loader.load(1).then(result => console.log('1', result))
loader.load(2).then(result => console.log('2', result))

setTimeout(() => {
  loader.load(3).then(result => console.log('3', result))
  loader.load(4).then(result => console.log('4', result))
}, 2000)

• Push a promise to _keys
• Promises in the _key array will resolve items in the result array
• Return the promise pushed to _keys, which corresponds to the load() function call for this key
• The timeout will only NOT be cleared for the last time this function is called. Therefore _batchLoadingFn will only be called once

For testing sake, the DataLoader is initialized with a batching function that will just double the key per result.