Asynchronous

JavaScript / NodeJS

Promises

• Use promises whenever you are using asynchronous or blocking code
• resolve maps to then and reject maps to catch for all practical purposes
• Make sure to write both .catch and .then methods for all the promises
• If something needs to be done in both cases use finally
• We only get one shot at mutating each promise
• We can add multiple handlers to a single promise
• The return type of all the methods in the Promise object, regardless of whether they are static methods or prototype methods, is again a Promise
• In Promise.all, the order of the promises are maintained in the values variable, irrespective of which promise was first resolved

Async

• async functions return a promise
• async functions use an implicit Promise to return results
  • Even if you don’t return a promise explicitly, the async function makes sure that your code is passed through a promise
• await blocks the code execution within the async function, of which it (await statement) is a part
• There can be multiple await statements within a single async function
• When using async await, make sure you use try catch for error handling
• Be extra careful when using await within loops and iterators
  • You might fall into the trap of writing sequentially-executing code when it could have been easily done in parallel
• await is always for a single Promise
• Promise creation starts the execution of asynchronous functionality
• await only blocks the code execution within the async function
  • It only makes sure that the next line is executed when the promise resolves
  • So, if an asynchronous activity has already started, await will not have any effect on it

async library

npm i async

async.map(["foo.txt", "bar.txt"], fs.stat, (error, results) => {
  console.log(results);
});

async.parallel(
  [
    (callback) => {
      setTimeout(callback, 1000);
    },
    (callback) => {
      setTimeout(callback, 1000);
    },
  ],
  (error, results) => {
    console.log("I took 2 seconds.");
  }
);

async.waterfall(
  [
    (callback) => {
      setTimeout(callback, 1000);
    },
    (callback) => {
      setTimeout(callback, 1000);
    },
  ],
  (error, results) => {
    console.log("I took 2 seconds.");
  }
);