node-singleflight/lib/do.js

provides a duplicate function call suppression

const flights = new Map()

// Do executes and returns the results of the given function, making
// sure that only one execution is in-flight for a given key at a
// time. If a duplicate comes in, the duplicate caller waits for the
// original to complete and receives the same results.
//
// Copy from golang pkg sync/singleflight
//
function Do (key, func) {
  if (flights.has(key)) {
    return flights.get(key)
  }

  let resolveOperation
  let rejectOperation
  const operation = new Promise((resolve, reject) => {
    resolveOperation = resolve
    rejectOperation = reject
  })

  const cleanup = () => {
    if (flights.get(key) === flight) {
      flights.delete(key)
    }
  }
  const flight = operation.then(
    (value) => {
      cleanup()
      return value
    },
    (error) => {
      cleanup()
      throw error
    }
  )

  flights.set(key, flight)

  try {
    resolveOperation(func())
  } catch (error) {
    rejectOperation(error)
  }

  return flight
}

module.exports = Do