Example of providing error-first callback mechanism by default but also a chained .promise() method

callback-or-promise.js

// Example from https://nickmeldrum.com/blog/intriguing-reason-node-not-promises-by-default
const doS3GetObject = params => {
    if (someErrorHappens) {
      throw new Error('uh oh, we borked')
    }
    else {
      return 'we done good' // imagine we actually did the work!
    }
}

const s3 = {
  getObject: (params, callback) => {
    if (callback) {
      try {
        callback(null, doS3GetObject(params))
      }
      catch (e) {
        callback(e)
      }
    }
    return {
      promise: () => new Promise((resolve, reject) => {
          try {
            resolve(doS3GetObject(params));
          } catch (e) {
            reject(e)
          }
        }),
    }
  },
}