Promise Throttler for Async Function

promise-throttler.js

// Example refreshAuth fn that might call Google
function refreshAuth (oldToken, done) {
  console.log('actually refreshing the old token', oldToken)
  setTimeout(() => {
    done(null, getNewToken())
  }, 2000)
}

// Pretend we're issuing new tokens
let token = 1
function getNewToken () {
  return token++
}

// Pretty generic module to throttle an async function
function promiseThrottler (myAsyncFn) {
  let activePromise = null

  // return a wrapper to make sure only one person calls refreshAuthFn at a time
  return function (...args) {
    console.log('promise throttler wrapper')
    let done = args.pop()

    // No active promise, create one
    if (!activePromise) {
      console.log('creating promise')
      // Setup promise, call refreshAuth fn and resolve or reject based on response
      activePromise = new Promise((resolve, reject) => {
        // Define a callback for when we actually call myAsyncFn
        let myAsyncFnCallback = (err, ...response) => {
          // Reject the promise, and pass along the error and response
          if (err) {
            return reject(err)
          }

          // Resolve the promise and pass along arguments (not error)
          resolve(response)
        }

        // Kick off refreshAuth call here passing along args and a callback to resolve/reject promise
        myAsyncFn.apply(null, args.concat([myAsyncFnCallback]))
      })
    }

    // Wait for promise to resolve, and call done callback
    activePromise
      .then(resultArgs => {
        // call done with a null error and rest of args
        done(null, ...resultArgs)
      })
      .catch(err => {
        done(err)
      })
      // Poor mans Promise.finally()
      .then(() => {
        activePromise = null
      })
  }
}

// You could use this in another place like so:

let throttledRefreshAuth = promiseThrottler(refreshAuth)

let myToken = getNewToken()
function gotNewToken (err, newToken) {
  if (err) {
    return console.error('error refreshing tokne: ', err)
  }

  myToken = newToken
  console.log('new token: ', newToken)
}

throttledRefreshAuth(myToken, gotNewToken)
throttledRefreshAuth(myToken, gotNewToken)
throttledRefreshAuth(myToken, gotNewToken)

// This should get handled in the first round of refreshes
setTimeout(() => {
  throttledRefreshAuth(myToken, gotNewToken)
}, 1000)

// Wait awhile and grab more new tokens
setTimeout(() => {
  console.log('something comes along later to refresh the token...')
  throttledRefreshAuth(myToken, gotNewToken)
  throttledRefreshAuth(myToken, gotNewToken)
}, 5000)