Rate limiting a Promise returning function

index.js

// heavily inspired by https://github.com/JMPerez/promise-throttle

const throttle = (f, rate) => {
  // rate is # of calls/sec
  const period = 1/rate * 1000 // minimum elapsed time (in ms) between calls (sec/call * 1000ms/sec)
  const queue = [ ] // queued function calls
  let last = 0 // last time `f` was called

  const applyNext = () => {
    if (queue.length === 0) return
    const now = new Date()
    const since = now - last // elapsed time since last call
    if (since >= period) {
      last = now
      const { f, args, resolve, reject } = queue.shift()
      f(...args).then(resolve).catch(reject)
    } else {
      setTimeout(applyNext, period - since) // try again at start of next period
    }
  }

  return (...args) => new Promise((resolve, reject) => {
    queue.push({ f, args, resolve, reject })
    applyNext()
  })
}

// ...

const apiCall = throttle(async (query) => {
  // ...
}, 50)

const main = async () => {
  const queries = [ 'a', 'b', 'c' ]
  const results = await Promise.all(queries.map(apiCall))
}
main()