JavaScript is synchronous. This means that it will execute your code block by order after hoisting. Before the code executes, var and function declarations are “hoisted” to the top of their scope.

This is an example of a synchronous code:

console.log('1') console.log('2') console.log('3')

This code will reliably log “1 2 3".

Asynchronous requests will wait for a timer to finish or a request to respond while the rest of the code continues to execute. Then when the time is right a callback will spring these asynchronous requests into action.

This is an example of an asynchronous code:

console.log('1') setTimeout(function afterTwoSeconds() {

console.log('2')

}, 2000) console.log('3')

This will actually log “1 3 2”, since the “2” is on a setTimeout which will only execute, by this example, after two seconds. Your application does not hang waiting for the two seconds to finish. Instead it keeps executing the rest of the code and when the timeout is finished it returns to afterTwoSeconds.

You may ask “Why is this useful?” or “How do I get my async code to become sync?”. Hopefully I can show you the answers.

“The problem”

Let us say our goal is to search for a GitHub user and get all the repositories of that user. The thing is we don’t know the exact name of the user. So we have to list all the users with similar name and their respective repositories.

Doesn’t need to super fancy, something like this

So much style, wow! This is the “fiddle”

In these examples the request code will use XHR (XMLHttpRequest). You can replace it with jQuery $.ajax or the more recent native approach called fetch . Both will give you the promises approach out of the gate.

It will be slightly changed depending on your approach but as a starter:

// url argument can be something like ' https://api.github.com/users/daspinola/repos function request(url) {

const xhr = new XMLHttpRequest();

xhr.timeout = 2000;

xhr.onreadystatechange = function(e) {

if (xhr.readyState === 4) {

if (xhr.status === 200) {

// Code here for the server answer when successful

} else {

// Code here for the server answer when not successful

}

}

}

xhr.ontimeout = function () {

// Well, it took to long do some code here to handle that

}

xhr.open('get', url, true)

xhr.send();

}

Remember that in these examples the important part is not what the end result of the code is. Instead your goal should be to understand the differences of the approaches and how you can leverage them for your development.

Callback

You can save a reference of a function in a variable when using JavaScript. Then you can use them as arguments of another function to execute later. This is our “callback”.

One example would be:

// Execute the function "doThis" with another function as parameter, in this case "andThenThis". doThis will execute whatever code it has and when it finishes it should have "andThenThis" being executed. doThis(andThenThis) // Inside of "doThis" it's referenced as "callback" which is just a variable that is holding the reference to this function function andThenThis() {

console.log('and then this')

} // You can name it whatever you want, "callback" is common approach function doThis(callback) {

console.log('this first')



// the '()' is when you are telling your code to execute the function reference else it will just log the reference callback()

}

Using the callback to solve our problem allows us to do something like this to the request function we defined earlier:

function request(url, callback) {

const xhr = new XMLHttpRequest();

xhr.timeout = 2000;

xhr.onreadystatechange = function(e) {

if (xhr.readyState === 4) {

if (xhr.status === 200) {

callback(null, xhr.response)

} else {

callback(xhr.status, null)

}

}

}

xhr.ontimeout = function () {

console.log('Timeout')

}

xhr.open('get', url, true)

xhr.send();

}

Our function for the request will now accept a callback so that when a request is made it will be called in case of error and in case of success.

const userGet = ` https://api.github.com/search/users?page=1&q=daspinola&type=Users` request(userGet, function handleUsersList(error, users) {

if (error) throw error

const list = JSON.parse(users).items list.forEach(function(user) {

request(user.repos_url, function handleReposList(err, repos) {

if (err) throw err

// Handle the repositories list here

})

})

})

Breaking this down:

We make a request to get a user’s repositories

After the request is complete we use callback handleUsersList

If there is no error then we parse our server response into an object using JSON.parse

Then we iterate our user list since it can have more than one

For each user we request their repositories list.

We will use the url that returned per user in our first response

We call repos_url as the url for our next requests or from the first response

For each user we request their repositories list. We will use the url that returned per user in our first response We call as the url for our next requests or from the first response When the request has completed the callback, we will call

This will handle either its error or the response with the list of repositories for that user

Note: Sending the error first as parameter is a common practice especially when using Node.js.

A more “complete” and readable approach would be to have some error handling. We would keep the callback separate from the request execution.

Something like this:

try {

request(userGet, handleUsersList)

} catch (e) {

console.error('Request boom! ', e)

} function handleUsersList(error, users) {

if (error) throw error

const list = JSON.parse(users).items list.forEach(function(user) {

request(user.repos_url, handleReposList)

})

} function handleReposList(err, repos) {

if (err) throw err



// Handle the repositories list here

console.log('My very few repos', repos)

}

This ends up having problems like racing and error handling issues. Racing happens when you don’t control which user you will get first. We are requesting the information for all of them in case there is more than one. We are not taking an order into account. For example, user 10 can come first and user 2 last. We have a possible solution later in the article.

The main problem with callbacks is that maintenance and readability can become a pain. It sort of already is and the code does hardly anything. This is known as callback hell which can be avoided with our next approach.

Image taken from here. Callback hell at its best.

Promises

Promises you can make your code more readable. A new developer can come to the code base and see a clear order of execution to your code.

To create a promise you can use:

const myPromise = new Promise(function(resolve, reject) {



// code here



if (codeIsFine) {

resolve('fine')

} else {

reject('error')

} }) myPromise

.then(function whenOk(response) {

console.log(response)

return response

})

.catch(function notOk(err) {

console.error(err)

})

Let us decompose it:

A promise is initialized with a function that has resolve and reject statements

that has and statements Make your async code inside the Promise function

resolve when everything happens as desired

Otherwise reject

function when everything happens as desired Otherwise When a resolve is found the .then method will execute for that Promise

When a reject is found the .catch will be triggered

Things to bear in mind:

resolve and reject only accept one parameter

resolve(‘yey’, ‘works’) will only send ‘yey’ to the .then callback function

and only accept one parameter will only send ‘yey’ to the callback function If you chain multiple .then

Add a return if you want the next .then value not to be undefined

Add a if you want the next value not to be When a reject is caught with .catch if you have a .then chained to it

It will still execute that .then

You can see the .then as an “always executes” and you can check an example in this comment

is caught with if you have a chained to it It will still execute that You can see the as an “always executes” and you can check an example in this comment With a chain on .then if an error happens on the first one

It will skip subsequent .then until it finds a .catch

if an error happens on the first one It will skip subsequent until it finds a A promise has three states

pending

When waiting for a resolve or reject to happen

resolved

rejected

or to happen Once it’s in a resolved or rejected state

It cannot be changed

Note: You can create promises without the function at the moment of declarations. The way that I’m showing it is only a common way of doing it.

“Theory, theory, theory…I’m confused” you may say.

Let’s use our request example with a promise to try to clear things up:

function request(url) {

return new Promise(function (resolve, reject) {

const xhr = new XMLHttpRequest();

xhr.timeout = 2000;

xhr.onreadystatechange = function(e) {

if (xhr.readyState === 4) {

if (xhr.status === 200) {

resolve(xhr.response)

} else {

reject(xhr.status)

}

}

}

xhr.ontimeout = function () {

reject('timeout')

}

xhr.open('get', url, true)

xhr.send();

})

}

In this scenario when you execute request it will return something like this:

A promise pending to be resolved or rejected

const userGet = ` https://api.github.com/search/users?page=1&q=daspinola&type=Users` const myPromise = request(userGet) console.log('will be pending when logged', myPromise) myPromise

.then(function handleUsersList(users) {

console.log('when resolve is found it comes here with the response, in this case users ', users) const list = JSON.parse(users).items

return Promise.all(list.map(function(user) {

return request(user.repos_url)

}))

})

.then(function handleReposList(repos) {

console.log('All users repos in an array', repos)

})

.catch(function handleErrors(error) {

console.log('when a reject is executed it will come here ignoring the then statement ', error)

})

This is how we solve racing and some of the error handling problems. The code is still a bit convoluted. But its a way to show you that this approach can also create readability problems.

A quick fix would be to separate the callbacks like so:

const userGet = ` https://api.github.com/search/users?page=1&q=daspinola&type=Users` const userRequest = request(userGet) // Just by reading this part out loud you have a good idea of what the code does

userRequest

.then(handleUsersList)

.then(repoRequest)

.then(handleReposList)

.catch(handleErrors) function handleUsersList(users) {

return JSON.parse(users).items

} function repoRequest(users) {

return Promise.all(users.map(function(user) {

return request(user.repos_url)

}))

} function handleReposList(repos) {

console.log('All users repos in an array', repos)

} function handleErrors(error) {

console.error('Something went wrong ', error)

}

By looking at what userRequest is waiting in order with the .then you can get a sense of what we expect of this code block. Everything is more or less separated by responsibility.

This is “scratching the surface” of what Promises are. To have a great insight on how they work I cannot recommend enough this article.

Generators

Another approach is to use the generators. This is a bit more advance so if you are starting out feel free to jump to the next topic.

One use for generators is that they allow you to have async code looking like sync.

They are represented by a * in a function and look something like:

function* foo() {

yield 1

const args = yield 2

console.log(args)

}

var fooIterator = foo() console.log(fooIterator.next().value) // will log 1

console.log(fooIterator.next().value) // will log 2 fooIterator.next('aParam') // will log the console.log inside the generator 'aParam'

Instead of returning with a return , generators have a yield statement. It stops the function execution until a .next is made for that function iteration. It is similar to .then promise that only executes when resolved comes back.

Our request function would look like this:

function request(url) {

return function(callback) {

const xhr = new XMLHttpRequest();

xhr.onreadystatechange = function(e) {

if (xhr.readyState === 4) {

if (xhr.status === 200) {

callback(null, xhr.response)

} else {

callback(xhr.status, null)

}

}

}

xhr.ontimeout = function () {

console.log('timeout')

}

xhr.open('get', url, true)

xhr.send()

}

}

We want to have the url as an argument. But instead of executing the request out of the gate we want it only when we have a callback to handle the response.

Our generator would be something like:



const userGet = `



const users = yield request(userGet)



yield



for (let i = 0; i<=users.length; i++) {

yield request(users[i].repos_url)

}

} function* list() {const userGet = ` https://api.github.com/search/users?page=1&q=daspinola&type=Users` const users = yield request(userGet)yieldfor (let i = 0; i<=users.length; i++) {yield request(users[i].repos_url)

It will:

Wait until the first request is prepared

is prepared Return a function reference expecting a callback for the first request

Our request function accepts a url

and returns a function that expects a callback

reference expecting a for the first Our function accepts a and returns a that expects a Expect a users to be sent in the next .next

to be sent in the next Iterate over users

Wait for a .next for each of the users

for each of the Return their respective callback function

So an execution of this would be:

try {

const iterator = list()

iterator.next().value(function handleUsersList(err, users) {

if (err) throw err

const list = JSON.parse(users).items



// send the list of users for the iterator

iterator.next(list)



list.forEach(function(user) {

iterator.next().value(function userRepos(error, repos) {

if (error) throw repos // Handle each individual user repo here

console.log(user, JSON.parse(repos))

})

})

})

} catch (e) {

console.error(e)

}

We could separate the callback functions like we did previously. You get the deal by now, a takeaway is that we now can handle each individual user repository list individually.

I have mixed felling about generators. On one hand I can get a grasp of what is expected of the code by looking at the generator.

But its execution ends up having similar problems to the callback hell.

Like async/await, a compiler is recommended. This is because it isn’t supported in older browser versions.

Also it isn’t that common in my experience. So it may generate confusing in codebases maintained by various developers.

An awesome insight of how generators work can be found in this article. And here is another great resource.

Async/Await

This method seems like a mix of generators with promises. You just have to tell your code what functions are to be async . And what part of the code will have to await for that promise to finish.

sumTwentyAfterTwoSeconds(10)

.then(result => console.log('after 2 seconds', result)) async function sumTwentyAfterTwoSeconds(value) {

const remainder = afterTwoSeconds(20)

return value + await remainder

} function afterTwoSeconds(value) {

return new Promise(resolve => {

setTimeout(() => { resolve(value) }, 2000);

});

}

In this scenario:

We have sumTwentyAfterTwoSeconds as being an async function

as being an async function We tell our code to wait for the resolve or reject for our promise function afterTwoSeconds

or for our promise function It will only end up in the .then when the await operations finish

In this case there is only one

Applying this to our request we leave it as a promise as seen earlier:

function request(url) {

return new Promise(function(resolve, reject) {

const xhr = new XMLHttpRequest();

xhr.onreadystatechange = function(e) {

if (xhr.readyState === 4) {

if (xhr.status === 200) {

resolve(xhr.response)

} else {

reject(xhr.status)

}

}

}

xhr.ontimeout = function () {

reject('timeout')

}

xhr.open('get', url, true)

xhr.send()

})

}

We create our async function with the needed awaits like so:



const userGet = `



const users = await request(userGet)

const usersList = JSON.parse(users).items



usersList.forEach(async function (user) {

const repos = await request(user.repos_url)



handleRepoList(user, repos)

})

} async function list() {const userGet = ` https://api.github.com/search/users?page=1&q=daspinola&type=Users` const users = await request(userGet)const usersList = JSON.parse(users).itemsusersList.forEach(async function (user) {const repos = await request(user.repos_url)handleRepoList(user, repos)}) function handleRepoList(user, repos) {

const userRepos = JSON.parse(repos)



// Handle each individual user repo here console.log(user, userRepos)

}

So now we have an async list function that will handle the requests. Another async is needed in the forEach so that we have the list of repos for each user to manipulate.

We call it as:

list()

.catch(e => console.error(e))

This and the promises approach are my favorites since the code is easy to read and change. You can read about async/await more in depth here.

A downside of using async/await is that it isn’t supported in the front-end by older browsers or in the back-end. You have to use the Node 8.

You can use a compiler like babel to help solve that.

“Solution”

You can see the end code accomplishing our initial goal using async/await in this snippet.

A good thing to do is to try it yourself in the various forms referenced in this article.

Conclusion

Depending on the scenario you might find yourself using:

async/await

callbacks

mix

It’s up to you what fits your purposes. And what lets you maintain the code so that it is understandable to others and your future self.

Note: Any of the approaches become slightly less verbose when using the alternatives for requests like $.ajax and fetch .

Let me know what you would do different and different ways you found to make each approach more readable.

This is Article 11 of 30. It is part of a project for publishing an article at least once a week, from idle thoughts to tutorials. Leave a comment, follow me on Diogo Spínola and then go back to your brilliant project!