Different mechanisms for making async work sequential.

sequential-promises.js

// code for my devtip on 12 June 2018:
// https://www.youtube.com/watch?v=0wiM3jW1DVY&list=PLV5CVI1eNcJgCrPH_e6d57KRUTiDZgs0u
const loadFile = file => {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(`contents of ${file}`)
    }, 500)
  })
}
const files = ['01.md', '02.md', '03.md', '04.md', '05.md']

const allPromises = files.map(file => loadFile(file))
Promise.all(allPromises).then(result => {
  console.log('all', result)
})

let currentPromise = Promise.resolve([])
files.forEach(file => {
  currentPromise = currentPromise.then(allContents => {
    return loadFile(file).then(contents => {
      allContents.push(contents)
      return allContents
    })
  })
})
currentPromise.then(result => {
  console.log('forEach', result)
})

files.reduce((previousPromise, file) => {
  return previousPromise.then(allContents => {
    return loadFile(file).then(contents => {
      allContents.push(contents)
      return allContents
    })
  })
}, Promise.resolve([])).then(result => {
  console.log('reduce', result)
})

;(async () => {
  const allContents = []
  for (file of files) {
    allContents.push(await loadFile(file))
  }
  console.log('async', allContents)
})()

node 10 allows yet another variant

function* loadFiles() {
  for (file of files) {
    yield loadFile(file);
  }
}

(async () => {
  const allContents = [];
  for await (content of loadFiles()) {
    allContents.push(content);
  }
  console.log('for await', allContents);
})();