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Asynchronous Javascript Programming

Javascript event loop

Javascript whether it runs in the browser or on the server executes an event loop. Javascript runs as a single process and handles those events.

asynchronous programming

Callback Functions

Keep going – don’t wait for me.

Callback example using setTimeout

/* CALLBACK - 
setTimeout(function, milliseconds); */
function getData(ms, query) {
	setTimeout(function() {
		console.log(query);
	}, ms);
}
function query() {
	getData(3000, "Unicorn Store");
	console.log("A1");
	getData(1000, "Captain Marvel");
	console.log("A2");
}
query();

Callback example using processFile

// callback example
var fs = require("fs");
function processFile(filename) {
	fs.readFile(filename, { encoding: "utf8" }, (error, data) => {
		if (error) {
			console.log(error);
			return;
		}
		//now we have the data
		console.log("ProcessingComplete");
	});
}
console.log("A1");
processFile("/Users/raz/data/glove.6B/glove.6B.50d.txt");
console.log("A2");

Some points to remember:

  • Writing asynchronous code with callbacks requires a fair amount of code. As we will see in a second there are easier, less verbose ways of writing asynch. functions.
  • Aysynchronous code is infectious. Recursive.

Basic Synchronous Example

/* demo of synchronous calls 
   using a fake db query */

function sleep(ms) {
	const start = Date.now();
	while (Date.now() - start < ms) {}
}

function dbQuery(q, ms) {
	sleep(ms);
	if (q == "SELECT unicorn") {
		return "Unicorn Store";
	} else {
		return "Captain Marvel";
	}
}

function queryDB(query, ms) {
	let result = dbQuery(query, ms);
	console.log(result);
	return;
}

function doIt() {
	queryDB("SELECT unicorn", 3000);
	console.log("B");
	queryDB("SELECT captain marvel", 1000);
	console.log("C");
}

doIt();

Promises Example

function dbQuery(q, ms) {
	return new Promise(resolve => {
		setTimeout(() => {
			if (q == "SELECT unicorn") {
				resolve("Unicorn Store");
			} else {
				resolve("Captain Marvel");
			}
		}, ms);
	});
}

function queryDB(query, ms) {
	dbQuery(query, ms)
		.then(result => {
			console.log(result);
		})
		.catch(error => console.log(error));
}

function doIt() {
	queryDB("SELECT unicorn", 3000);
	console.log("B");
	queryDB("SELECT captain marvel", 1000);
	console.log("C");
}

doIt();

Async and Await

function dbQuery(q, ms) {
	return new Promise(resolve => {
		setTimeout(() => {
			if (q == "SELECT unicorn") {
				resolve("Unicorn Store");
			} else {
				resolve("Captain Marvel");
			}
		}, ms);
	});
}

async function queryDB(query, ms) {
	let result = await dbQuery(query, ms);
	console.log(result);
	return;
}

function doIt() {
	queryDB("SELECT unicorn", 3000);
	console.log("B");
	queryDB("SELECT captain marvel", 1000);
	console.log("C");
}

doIt();
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