Adds a meterMap method to an array to help throttle requests

addMetering

// runs a promise that resolves after time in milliseconds.
// usage `await pause(1000)`
function pause = time => new Promise(resolve => setTimeout(resolve, time));

// adds a "meter" method to an array
// a meter is like the freeway metering you see near onramps during peak hours.
// It works basically like a Promise.all(arr.map()) function, except that each
// mapped item waits `waitTime` milliseconds before actually starting. This helps
// throttle throughput to webservices, reducing the chance that they will not be 
// able to keep up with the concurrent load.
// examples:
// const myVals = await addMetering(arr, 1000).meterMap(async (item, idx, arr) => { ... do stuff }
// OR 
// addMetering(myArr);
// myArr.meterMap(cb, 1500).then(() => do stuff);
function addMetering(arr, waitTime) {
  if(Array.isArray(arr)) {
    arr.meterMap = async (callback, wait = waitTime) => {
      const result = [];
      for(let x = 0; x < arr.length; x++) {
        await pause(wait);
        result.push(callback(arr[x], x, arr));
      }
      
      return Promise.all(result);
    }
  } else {
    throw new TypeError('The first argument must be an array');
  }
  
  return arr;
}