loop-promise.js

const urls = [
  'https://google.com',
  'http://blainesch.com',
  'https://github.com',
]

let follower = Promise.resolve()

for (let currentUrl of urls) {
  follower = follower.then(() => {
    return new Promise((resolve) => {
      console.log('processing', currentUrl)
      setTimeout(resolve, 1000)
    })
  })
}

recursive-promise.js

const urls = [
  'https://google.com',
  'http://blainesch.com',
  'https://github.com',
]

const process = (urls) => {
  const currentUrl = urls.shift()
  return new Promise((resolve) => {
    console.log('processing', currentUrl)
    setTimeout(resolve, 1000)
  }).then(() => {
    if (urls.length > 0) {
      return process(urls)
    }
  })
}

process(urls.slice(0)).then(() => {
  console.log('done')
})

oooh.. I like the the top loop! it is super concise, and it seems like the recursive one uses the same mechanism, and is destructive to the URL array, which we might want to reuse over and over. Which one do you like best?

let foo = Promise.resolve()

const urls = [
  'https://google.com',
  'http://blainesch.com',
  'https://github.com',
]

let timeout = (time) => {
  return new Promise((resolve) => {
    setTimeout(resolve, time)
  })
}

for (let url of urls) {
  foo = foo.then(() => {
    console.log(url)
    return timeout(1000)
  })
}

Abstracted timeout to a function.