Promise example - chaining and parallel execution

promise-example.js

// Promises example
//

// just a helper, which returns a promise that resolves after
// 500-1000 milliseconds with whatever `args` were passed in
function fakeKnex(...args) {
  return new Promise((resolve, reject) => {
    setTimeout(
      function() { resolve(...args) },
      500 + (Math.floor(Math.random() * 500)))
  })
}

// example items
const ITEMS = [
  { orderItem: 1 },
  { orderItem: 2 },
  { orderItem: 3 },
  { orderItem: 4 },
  { orderItem: 5 },
]


// TIPS:
// - Promises in sequence: return value of `then` callback is another promise.
// - Promises in parallel: use `Promise.all`

// "insert user" with promise-chaining and Promise.all
fakeKnex({ some: 'user' })
  .then(userResult => {
    console.log(userResult)
    // insert order
    return fakeKnex({ some: 'order' })
  })
  .then(orderResult => {
    console.log(orderResult)
    // Promise.all(promises) - takes an array of promises, and returns a new
    // promise, which resolves only when each given promise has resolved.
    // It resolves with an array of values, one for each promise.
    return Promise.all(ITEMS.map(item => fakeKnex(item)))
  })
  .then(itemsResults => {
    console.log('order items:')
    itemsResults.forEach(result => {
      console.log(result)
    })
    console.log("Redirect to /")
  })
  .catch(console.error)


// the original version of the above code:
if (false) {
  fakeKnex({ some: 'user' })
    .then(userResult => {
      console.log(userResult)
      // insert order
      fakeKnex({ some: 'order' })
        .then(orderResult => {
          console.log(orderResult)
          // not sure how to do in parallel:
          ITEMS.forEach(item => {
            // not able to check the results
            fakeKnex(item).then(() => {})
          })
          // this happens before the items are actually added:
          console.log("Redirect to /")
        })
    })
    // no catch()
}