From Callbacks to Async Await

December 09, 2017


I built the same program 4 different ways. I started with callbacks, moved on to Promises, used generators, and finished up with async/await.

The program:

  1. Makes a request to Github’s users endpoint
  2. Pulls back my Github profile
  3. Logs the response

Here’s what I came up with.


I struggled to make an HTTP request using callbacks. I’ve mostly used Promises to write asynchronous JavaScript. I generally use axios or fetch in my applications, which are both Promise-based.

I ended up having to turn to XMLHTTPRequest for this version of the application, which I’ve never used before!

const makeHTTPRequest = (url, methodType, callback) => {
  const xhr = new XMLHttpRequest();, url, true);
  xhr.onreadystatechange = () => {
    if (xhr.readyState === 4 && xhr.status === 200) {

const getLogin = (response) => {

const url = "";
makeHTTPRequest(url, "GET", getLogin);

I’ll walk you through what this code does:

  1. I define a function called makeHTTPRequest. It’s designed to actually make the request to Github.
  2. I pass three methods to makeHTTPRequest, url, methodType, and callback. url is the endpoint I want to hit. methodType is the HTTP method I want to use. And callback is the function I want to call when I actually get a response back from Github.
  3. I define a function called getLogin and pass it response as an argument. The function takes the response I receive from Github, and parses it to JSON. Then, it logs the parsed response.
  4. I pass getLogin into makeHTTPRequest as callback. That means getLogin will take the response from Github has it’s argument.


After achieving my goals with callbacks, I attempted with promises. This felt straightforward, since I’ve done it many times before.

const makeHTTPRequest = (username) => {
  const url = "" + username;
    .then((response) => response.json())
    .then((response) => console.log(response));

  1. I defined a function called makeHTTPRequest and passed it a username.
  2. I used fetch to make a request to Github.
  3. I use .then() to wait for the request to Github to complete, and then convert the response to JSON.
  4. I log the response


This was my first foray into generators. The syntax and concept were totally foreign to me upon approaching this challenge.

function* getUser(username) {
  const uri = "" + username;
  const response = yield fetch(uri);
  const parsedResponse = yield response.json();

  1. I define getUser and say that it’s a generator by using the * syntax. I pass in username as an argument.
  2. I create a variable called response and set it equal to response I receive from Github after making an HTTP request using fetch. The important piece of line 3 is that I use the keyword yield. yield is telling my program that I do want to set response equal to the response I get back from Github, but only after the request is completed.
  3. I follow the same pattern again when I set parsedResponse equal to response.json(). I have to wait for the Promise to resolve before I can set my variable. If I don’t use yield, when I try to log parsedResponse, I get: Promise {<pending>} back.


Finally, I wrote the application once more using ES7 async/await. Since I did this after building the same application with generators, it became really obvious how async/await is built on top of generators.

const getUser = async (username) => {
  const uri = "" + username;
  const response = await fetch(uri);
  const parsedResponse = await response.json();

  1. I define an async function called getUser that takes username as an argument.
  2. I created a variable called responseand set it equal to the response I receive from Github after making a request to the /users/:id endpoint. The key is that I use the await keyword to tell my program to wait for the request to resolve itself before setting response equal to the response I get back.
  3. I use the same pattern again on line 4.
  4. Then I log the parsed response.


This was a worthy endeavor for a few reasons. First of all, I hadn’t used callbacks much, so I didn’t really know how big of a deal Promises were. I also had been taking fetch and axios for granted, since they made making HTTP requests so much more straightforward.

I also had never used generators or async/await. Although I found async/await much easier to use than generators, it was helpful to see how async/await was built on top of generators. It was also fun to get my hands dirty with some ES7 syntax.

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