An example to show nested callback's complexity

callback-hell-example.js

function buyBread (input, callback) {
  gotoStore(input, (storeErr, storeResponse) => {
    if (storeErr) {
      return callback(storeErr, null)
    }
    buyBrownBread(input, (brownBreadErr, brownBreadResponse) => {
      if (brownBreadErr) {
        buyWhiteBread(input, (whiteBreadErr, whiteBreadResponse) => {
          if (whiteBreadErr) {
            return callback(whiteBreadErr, null)
          }
          callback(null, whiteBreadResponse)
        })
      } else {
        callback(null, brownBreadResponse)
      }
    })
  })
}