public/subjects/devops/kept-promise/README.md

kept-promise

Instructions

Things in your code do not always happen as fast as you would like.

Sometimes it is important to wait for a function to finish before keep going on. Other times you don't want your code to be blocking while waiting.

Create an asynchronous function processInfo in a file kept-promise.js. The function processInfo will take an asynchronous function as an input, and it will print the message Ok! if the return of the input function is zero or a multiple of two, Error! otherwise.

The following function can be used as an example of input function for processInfo:

const getImportantInfo = async () =>
  new Promise(resolve => resolve(Math.round(Math.random() * 10)))

Assume that your function will always get a valid input function

Example

The following script main.js can be used to test your function:

import processInfo from './kept-promise.js'

const getImportantInfo = () =>
  new Promise(resolve => resolve(Math.round(Math.random() * 10)))

console.log(await processInfo(getImportantInfo))

The output should be the following:

$  node main.js
Ok!
$  node main.js
Error!
$  node main.js
Ok!
$

Hints

• Asynchronous behavior, in the context of server-side JavaScript, refers to the ability of running code in a non-blockingly way, meaning for instance that the server can handle multiple requests simultaneously without waiting for a long-running task to complete, avoiding blocking the server responsiveness. It is essential to achieve scalable and functional back-end Node applications.

• A Promise is a special JS object that represent the result (succes or failure) of an asynchronous operation. This special object is usually used to "wrap" asynchronous operations. Promises can have three different states: pending - the asynchronous operation has not finished yet, fulfilled/resolved - the asynchronous operation has finished successfully - or rejected - the asynchronous operation has finished, but something went wrong. When defined from scratch, it is possible to define a resolve and reject callback function for a new Promise that will define the results of the success or failure of asynchronous operation happening inside the Promise.

• It is possible to wait for an asynchronous function with the keyword await. Alternatively, it can be clearer to use the method .then. Below an example of how to handle promises and, more generally, asynchronous operations.

const promise1 = new Promise((resolve, reject) => {
  resolve('Success!')
})

console.log(await promise1)
// Expected output: "Success!"

promise1.then(value => {
  console.log(value)
  // Expected output: "Success!"
})

Notions

• Introducing asynchronous JavaScript
• await
• Using promises
• async function
• then