Async

Examples from the Async presentation from October 2018. Here are the slides from that meetup.

Timers

Timer functions include setImmediate, setTimeout and setInterval.

setTimeout(function () {
    console.log('Print something in awhile.');
}, 2000); // This will print 'Print something in awhile.' in 2 seconds.

Error first Callbacks

Error first Callbacks are functions that do not return any value, but have a last parameter that take a function with two parameters. The first paramater is usually some sort of error, while the second parameter is the data that would have been normally returned in a Synchronous function.

const fs = require('fs');
const location = __dirname + '/sometext.txt';

fs.readFile(location, 'utf-8', function (err, result) {
    if (err) {
        console.error(err);
    }
    console.log(result);
});

This can also be separated into a separate function to make it more readable.

const fs = require('fs');
const location = __dirname + '/sometext.txt';

fs.readFile(location, 'utf-8', processFile);

function processFile(err, result) {
    if (err) {
        console.error(err);
    }
    console.log(result);
}

Generators

Generators can yield many results without have to complete execution until all of the data has been delivered. These function can be identified by having * between the function keyword and function name as shown in the example below.

function* myIterator() {
    yield 14;
    yield 22;
    yield 16;
    return 18;
}

const iterator = myIterator();
iterator.next().value; // 14
iterator.next().value; // 22
iterator.next().value; // 16 

Promises

Promises are a special object asynchornous type that are a kind of monad. Promises use a constructor that take a callback with a resolve and reject parameters. Errors are passed into the reject function callback, and succesful results are passed back through the resolve callback.

const promise = new Promise(function(resolve, reject) {
  if (/* everything turned out fine */) {
    resolve("Stuff worked!");
  }
  else {
    reject(Error("It broke"));
  }
});

promise.then(result => {
    console.log(result);
}).catch(err => {
    console.error(err);
});

Async/Await

The async and await keywords were added as part of the es2017 language specification. Any function can be turned into a promise by adding the async keyword to the beginning of the function. Any thenable promise can be resolved inside of that async function by placing the await keyword in front of the promise.

function resolveAfter2Seconds() {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve('resolved');
    }, 2000);
  });
}

async function asyncCall() {
  console.log('calling');
  var result = await resolveAfter2Seconds();
  console.log(result);
  // expected output: 'resolved'
}

asyncCall();

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