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One of the keys to writing a successful web application is being able to make dozens of AJAX calls per page.
Asynchronous programming used to be a challenge even for seasoned professionals, leading to aptly named phenomena like Callback Hell.
This is a typical asynchronous programming challenge, and how you choose to deal with asynchronous calls will, in large part, make or break your app, and by extension potentially your entire startup. Asynchronous programming is a part of our everyday work, but the challenge is often taken lightly and not considered at the right time.
History of Asychronous JavaScript
The ancient solution to synchronize these calls was via nested callbacks. This was a decent approach for simple asynchronous JavaScript tasks, but wouldn’t scale because of an issue called callback hell.
The code for the three simple tasks would look something like this:
const verifyUser = function(username, password, callback){
dataBase.verifyUser(username, password, (error, userInfo) => {
if (error) {
callback(error)
}else{
dataBase.getRoles(username, (error, roles) => {
if (error){
callback(error)
}else {
dataBase.logAccess(username, (error) => {
if (error){
callback(error);
}else{
callback(null, userInfo, roles);
}
})
}
})
}
})
};
Each function gets an argument which is another function that is called with a parameter that is the response of the previous action.
Too many people will experience brain freeze just by reading the sentence above. Having an application with hundreds of similar code blocks will cause even more trouble to the person maintaining the code, even if they wrote it themselves.
This example gets even more complicated once you realize that a database.getRoles
is another function that has nested callbacks.
const getRoles = function (username, callback){
database.connect((connection) => {
connection.query('get roles sql', (result) => {
callback(null, result);
})
});
};
In addition to having code that is difficult to maintain, the DRY principle has absolutely no value in this case. Error handling, for example, is repeated in each function and the main callback is called from each nested function.
More complex asynchronous JavaScript operations, such as looping through asynchronous calls, is an even bigger challenge. In fact, there is no trivial way of doing this with callbacks. This is why JavaScript Promise libraries like Bluebird and Q got so much traction. They provide a way to perform common operations on asynchronous requests that the language itself doesn’t already provide.
That’s where native JavaScript Promises come in.
JavaScript Promises
Promises were the next logical step in escaping callback hell. This method did not remove the use of callbacks, but it made the chaining of functions straightforward and simplified the code, making it much easier to read.
With Promises in place, the code in our asynchronous JavaScript example would look something like this:
const verifyUser = function(username, password) {
database.verifyUser(username, password)
.then(userInfo => dataBase.getRoles(userInfo))
.then(rolesInfo => dataBase.logAccess(rolesInfo))
.then(finalResult => {
//do whatever the 'callback' would do
})
.catch((err) => {
//do whatever the error handler needs
});
};
To achieve this kind of simplicity, all of the functions used in the example would have to be Promisified. Let’s take a look at how the getRoles
method would be updated to return a Promise
:
const getRoles = function (username){
return new Promise((resolve, reject) => {
database.connect((connection) => {
connection.query('get roles sql', (result) => {
resolve(result);
})
});
});
};
We have modified the method to return a Promise
, with two callbacks, and the Promise
itself performs actions from the method. Now, resolve
and reject
callbacks will be mapped to Promise.then
and Promise.catch
methods respectively.
You may notice that the getRoles
method is still internally prone to the pyramid of doom phenomenon. This is due to the way database methods are created as they do not return Promise
. If our database access methods also returned Promise
the getRoles
method would look like the following:
const getRoles = new function (userInfo) {
return new Promise((resolve, reject) => {
database.connect()
.then((connection) => connection.query('get roles sql'))
.then((result) => resolve(result))
.catch(reject)
});
};
Approach 3: Async/Await
The pyramid of doom was significantly mitigated with the introduction of Promises. However, we still had to rely on callbacks that are passed on to .then
and .catch
methods of a Promise
.
Promises paved the way to one of the coolest improvements in JavaScript. ECMAScript 2017 brought in syntactic sugar on top of Promises in JavaScript in the form of async
and await
statements.
They allow us to write Promise
-based code as if it were synchronous, but without blocking the main thread, as this code sample demostrates:
const verifyUser = async function(username, password){
try {
const userInfo = await dataBase.verifyUser(username, password);
const rolesInfo = await dataBase.getRoles(userInfo);
const logStatus = await dataBase.logAccess(userInfo);
return userInfo;
}catch (e){
//handle errors as needed
}
};
Awaiting Promise
to resolve is allowed only within async
functions which means that verifyUser
had to be defined using async function
.
However, once this small change is made you can await
any Promise
without additional changes in other methods.
Async – A Long Awaited Resolution of a Promise
Async functions are the next logical step in the evolution of asynchronous programming in JavaScript. They will make your code much cleaner and easier to maintain. Declaring a function as async
will ensure that it always returns a Promise
so you don’t have to worry about that anymore.
Why should you start using the JavaScript async
function today?
- The resulting code is much cleaner.
- Error handling is much simpler and it relies on
try
/catch
just like in any other synchronous code. - Debugging is much simpler. Setting a breakpoint inside a
.then
block will not move to the next.then
because it only steps through synchronous code. But, you can step throughawait
calls as if they were synchronous calls.