callback hell<\/a>. This is a situation when too many nested callbacks are used.<\/p>\n\n\n\nconst userVerification = function(uname, pword, callback){\n db.userVerification(uname, pword, (err, info) =>{\n if(err){\n callback(err)\n }\n\telse{\t\n\t db.fetchRoles(uname, (err, roles) =>{\t\n\t if(err) {\n\t\t callback(err)\n\t\t}\n\t\telse{\n\t\t callback(null, info, roles)\n\t\t}\n }\n\t}\n \n }\n}\n<\/code><\/pre>\n\n\n\nAs you can probably tell, the code above is complicated. First, the “userVerification” method is called for “info, next the “fetchRoles” method is called for “roles”. As these are database operations, they can take some time to complete, therefore we are leveraging asynchronous programming. It’s apparent that this could quickly get more complex, for example, if there needed to be more callbacks in the “userVerification” and “fetchRoles” functions. Many nested callbacks will cause problems in terms of complexity and maintainability of your code. To avoid the callback hell, there are two alternatives available to us in JavaScript, promises and async\/await.<\/p>\n\n\n\n
Promise<\/h2>\n\n\n\n Promises, just like in real life, are either fulfilled or broken. Promises in JavaScript work similarly. Let’s explore this further through an example. Sam’s birthday is January 10th, and today is January 1st. Sam’s father made a promise that he will buy Sam a brand new iPhone on his birthday which is 10 days later. From January 1st to the 9th, the promise made by Sam’s father is pending. Only on January 10th will Sam find out if the promise that his father made will be fulfilled. There can be two scenarios: Sam’s father fulfills the promise and Sam gets a brand new iPhone, or Sam’s father breaks the promise. Promises work similarly in JavaScript. A promise in JavaScript has three possible states – pending, fulfilled, and rejected.<\/p>\n\n\n\n
const promise = new Promise((resolve, reject) => {\n\n})\n<\/code><\/pre>\n\n\n\nA promise is created using the new keyword and Promise method. The Promise method has a callback as its parameter. The callback function, in turn, has two parameters, resolve and reject. Let’s take a closer look with a working example below.<\/p>\n\n\n\n
Resolving a promise<\/h2>\n\n\n\n Let’s assume that we have three functions in a program: add, double, and display. The add function returns the sum of two numbers in 5 seconds, the double function returns the double value of the sum in 3 seconds, and the display function returns the final message in 2 seconds. This is asynchronous but instead of using callback functions we are using promises.<\/p>\n\n\n\n
const add = (x, y) =>{\n return new Promise((resolve,reject) => {\n setTimeout(() =>{\n resolve(x + y)\n }, 5000 )\n })\t\t\n}\n\nconsole.log(add(10, 20));\n<\/code><\/pre>\n\n\n\nThe add function returns a promise, which has a setTimeout function.<\/p>\n\n\n\nconst add = (x, y) =>{\n console.log(\"Wait for 5 seconds...\")\n\n \n return new Promise((resolve,reject) => \n setTimeout(() =>{\n\n resolve(x + y)\n }, 5000 )\n })\t\t\n}\n\nadd(10, 20).then( value => {\n\tconsole.log(value);\n})\n<\/code><\/pre>\n\n\n\nThe then<\/strong> method is attached with the add function, which also has a callback function as the parameter. The promise returned by the add function is pending, and when it finishes (the timer of the setTimeout function ends), the promise is resolved. The then method is used to handle the promise.<\/p>\n\n\n\nThe parameter of the then method is nothing but the value we passed to the resolve function earlier in the setTimeout function.<\/p>\n\n\n\n
const display = (q) =>{\n console.log(\"Wait for 2 seconds...\")\n return new Promise((resolve,reject) => {\n\n setTimeout(() =>{\n resolve(\"The final value is: \" + q)\n }, 2000 )\n })\t\t\n}\n\nconst double = (z) =>{\n console.log(\"Wait for 3 seconds...\")\n return new Promise((resolve,reject) => {\n setTimeout(() =>{\n resolve(z * 2)\n }, 3000 )\n })\t\t\n}\n\nconst add = (x, y) =>{\n console.log(\"Wait for 5 seconds...\")\n return new Promise((resolve,reject) => {\n setTimeout(() =>{\n resolve(x + y)\n }, 5000 )\t\n })\t\t\n}\n\nadd(10, 20).then( value1 => {\n double(value1).then( value2 =>{\n\n display(value2).then( value3 =>{\n console.log(value3)\n })\n })\n\t\n})\n<\/code><\/pre>\n\n\n\nThe above code is the complete example discussed earlier. Let’s break this down:<\/p>\n\n\n\n
add(10, 20).then( value1 => {<\/code><\/pre>\n\n\n\nIn this line, the add function is called and it returns the sum of 10 and 20 in 5 seconds. “value1” is the sum of 10 and 20.<\/p>\n\n\n\n
add(10, 20).then( value1 => { \n double(value1).then( value2 =>{<\/code><\/pre>\n\n\n\nAfter waiting for 5 seconds, we have the sum, so we passed it to the double function. “value2” is returned by the double function after 3 seconds.<\/p>\n\n\n\n
add(10, 20).then( value1 => { \n double(value1).then( value2 =>{ \n display(value2).then( value3 =>{<\/code><\/pre>\n\n\n\nNext, after waiting 2 seconds, we have the double value, which we passed to the display function. “value3” is returned by the display function after 2 seconds.<\/p>\n\n\n\n
add(10, 20).then( value1 => {\n double(value1).then( value2 =>{\n display(value2).then( value3 =>{\n console.log(value3)\n\n })\n\n })\n\n})<\/code><\/pre>\n\n\n\nFinally, “value3” is printed in the console.<\/p>\n\n\n\nRejecting a promise<\/h2>\n\n\n\n Let’s assume we do not want to add the number if they are equal. In this situation, we would have to reject the promise. In order to do this, we can attach what is known as a “catch” method.<\/p>\n\n\n\n
const add = (x, y) =>{\n\n return new Promise((resolve,reject) => {\n if(x != y){\n setTimeout(() =>{\n resolve(x + y)\n }, 5000 )\n }\n else{\n reject(\"x and y are equal!\")\n }\n })\t\t\n}\n\nadd(10, 10).then( value1 => {\n console.log(value1);\n})\n.catch(err => {\n console.log(err);\n})\n<\/code><\/pre>\n\n\n\nThe promise is rejected when x is equal to y, therefore we have a catch method attached directly after the add statement. The catch method has a callback function as its parameter and the callback function, in turn, has a parameter, i.e. error which is displayed in the console.<\/p>\n\n\n\nAsync\/await<\/h2>\n\n\n\n Working with promises is complicated. In a real application, you would be handling multiple API calls, whereby several mechanisms would be in place to work with data and potentially multiple external sources. Due to its complexity, the concept of async\/await was revealed with ECMAScript 2017.<\/p>\n\n\n\n
Async\/await is an approach that uses promises in a better and more simplistic way. These are essentially keywords that help us synchronously write promise-based code, without blocking the main thread. In other words, we can write asynchronous code as it was synchronous, greatly simplifying the development process.<\/p>\n\n\n\n
A function declared with the “async” keyword is an async function that returns a promise. It does not matter what an async function is returning, it will always be a promise. Let’s take a look at some examples.<\/p>\n\n\n\n
async function add(x, y){\n return x + y\n}\n\nconsole.log(add(10,20))\n<\/code><\/pre>\n\n\n\n“add” is an async function. It returns the sum of x and y.<\/p>\n\n\n\nThis function returns a promise, but we only return the sum of x and y. With async, we can simply use the then<\/strong> method to unwrap the promise.<\/p>\n\n\n\nasync function add(x, y){\n return x + y;\t\n}\n\nadd(10,20).then( value => {\n console.log(value); \/\/30\n})<\/code><\/pre>\n\n\n\nWe can clearly see the difference async functions can make. Next, we’re going to look at await, but before we do that let’s take a look at the add<\/strong> function below.<\/p>\n\n\n\nconst add = (x, y) =>{\n\n console.log(\"Wait for 5 seconds...\")\n\n return new Promise((resolve,reject) => {\n\n \n setTimeout(() =>{\n\n \n \n resolve(x + y)\n\n \n }, 5000 )\n\t\n\n })\t\t\n}\n\nadd(10, 20).then( value => {\n\n console.log(value);\n})<\/code><\/pre>\n\n\n\nIn order to demonstrate async\/await, let’s modify the above code.<\/p>\n\n\n\n
The await<\/strong> keyword will help us reduce the complexity of the code. This keyword holds the JavaScript execution until the promise is settled.<\/p>\n\n\n\nfunction add(x, y){\n const pr = new Promise((resolve,reject) => {\n setTimeout(() =>{\n resolve(x + y)\n }, 5000 )\n })\t\n\n const sum = await pr; \n\n console.log(\"Sum of x and y:\", sum);\n}\n<\/code><\/pre>\n\n\n\nIn this function, a promise (pr) is created.<\/p>\n\n\n\n
const sum = await pr;<\/code><\/pre>\n\n\n\nThe execution will halt at this point without blocking the main thread. But we cannot use “await” in a normal function, it needs to be in an async function. The nice thing here is that we do not need the “then” method to unwrap the promise. The “await” keyword essentially handles everything for us!<\/p>\n\n\n\n
async function add(x, y){\n const pr = new Promise((resolve,reject) => {\n setTimeout(() =>{\n resolve(x + y)\n\t}, 5000 )\n })\t\n\t\n console.log(\"Waiting for 5 seconds...\");\n const sum = await pr; \n console.log(\"Sum of x and y:\", sum);\n}\n\nadd(10, 20)\n<\/code><\/pre>\n\n\n\nExecuting this code results in the following:<\/p>\n\n\n\nTo conclude, this is how we can use async\/await as the alternative to the traditional promise approach.<\/p>\n\n\n\n
Conclusion<\/h2>\n\n\n\n In this article, we covered everything regarding asynchronous JavaScript, starting from synchronous and asynchronous programming, how to handle asynchronous using callbacks, discussing callback hell and how to counter this using promises, and finally, the alternative to promises – async\/await. If are a JavaScript developer working with Node.js, understanding asynchronous JavaScript is essential. Hopefully you have gained an understanding of how JavaScript can support both synchronous and asynchronous programming, the execution context, call stack, message queue, and event loop. These are topics that are generally glossed over, but are essential to gaining a deeper knowledge of JavaScript development.<\/p>\n\n\n\n
For more on Asynchronous development with JavaScript, see: Asynchronous Programming :: Eloquent JavaScript<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"The term “asynchronous” is often associated with JavaScript, which can be a confusing concept, especially for beginners. This can become even more confusing because JavaScript is a synchronous language with asynchronous capabilities. In order to understand asynchronous development in general, we need to start with the difference between synchronous and asynchronous programming. In simple terms, […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[24],"tags":[],"post_series":[],"yoast_head":"\n
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