JS: Network Requests - Fetch

Learning Goals

• Understand the difference between synchronous and asynchronous operations
• Get familiar with the Promise object and the fetch API
• Know what a GET request does and how to use it
• Know what POST requests do and how to use them

Vocab

• Async / Asynchronous Executing code without blocking the execution of code after it
• AJAX Updating a webpage based on data from the network without reloading the whole thing
• The Promise Object - represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

What is a network request?

Let’s start by reviewing what a network request is and see it in action!

Making a Request

Each network request takes time - they’re expensive. Imagine if you had to wait for a webpage to load one thing at a time! It would not make for a great user experience.

Network requests are expensive no matter what we do. However, we can run them asynchronously, saving some time.

Asynchronous operations refer to things that can happen outside the normal order of execution. Network requests can be synchronous or asynchronous, but most modern applications do them asynchronously to improve performance / the user experience.

ES6: fetch()

Speaking of using the fetch API, let’s take a look at the docs on what it is!

fetch(resourceUrl, {/* object with `method`, `body`, and other optional properties */});

GET with Fetch

By default, fetch performs a GET request. This means that if we only add a resource url to the fetch call, we’ll try and GET information from that resource.

fetch("https://opentdb.com/api.php?amount=1&category=27&type=multiple");

Promises - The Quick Version

A Promise is an object that represents the eventual completion of an action.

A Promise will either be resolved upon completion, or rejected upon failure. We can use special methods for promises to determine what needs to happen in either of those scenarios:

• .then() runs upon the resolution of a promise and returns another promise
• .catch() runs upon the rejection of a failed promise and is used for error handling

What do I do with this “Promise {}"?

Diving into the returned promise reveals some information, such as its status and value, but nothing that’s too immediately useful. Instead we have to resolve it:

fetch("https://opentdb.com/api.php?amount=1&category=27&type=multiple")
  .then(response => console.log(response))

• What do you get when you log the response object? Take note of the properties there.
• There’s one problem: we can’t seem to get the data we want from the Response.body. How is data sent through requests and responses?

Parsing Our Response

Similar to what you did with localStorage, we’ll need to parse our response! We’ll need to use the Body.json() method that comes with fetch to parse it and call another .then().

From the docs, the .json() method returns “A promise that resolves with the result of parsing the body text as JSON. This could be anything that can be represented by JSON — an object, an array, a string, a number.

Let’s try it out!

fetch("https://opentdb.com/api.php?amount=1&category=27&type=multiple")
  .then(response => response.json())
  .then(data => console.log(data));

Lastly, we can add in a .catch() to account for any errors we may run into.

fetch("https://opentdb.com/api.php?amount=1&category=27&type=multiple")
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(err => /* do something else */);

Common Misconceptions

Let’s look at this code:

fetch('some_url')
.then(res => res.json())
.then(data => console.log(data))
.then(data => /* do something with data */) /* <--- this line won't work! */

Let’s think about these questions:

Big takeaways:

1. Its important to be aware of what each step is returning. Fetch returns a promise. Each .then() returns a promise. The callback function of the first .then() must return the json’d response object. Often this is happening implicitly as a one-line arrow function. However, if you open up curly braces to include a console log, that return is no longer happening implicitly so you need to explictly return the response.json() using the return keyword.
2. Console logs are great as you’re coding as a way to see what the data looks like that is coming through. BUT! Once you’re ready to actually do things with that data, you’ll want to remove your console logs OR have the console log in the same callback function as your logic, like this:

fetch('some_url')
.then(res => res.json())
.then(data => {
  console.log(data);
  /* do something with data */
})

POST with Fetch

What if we want to add information to a database?

If we want to use fetch to make any other kind of request, we’ll have to add an optional init object into the function call.

fetch(url, {
  method:"POST",
})

fetch(url, {
  method: 'POST',
  body: JSON.stringify(someDataToSend), // HTTP can only send and receive strings
  headers: {
  	'Content-Type': 'application/json'
  }
})
  .then(response => response.json())
  .then(data => /* do something with data */)
  .catch(err => /* do something with the error */);

Async JS

Let’s say we’re at a restaurant for a night out on the town…Here’s how the experience would go in each scenario:

• Synchronous: I order my food, everyone in the restaurant has to wait until I get my food before the next person can order.

• Asynchronous: I order my food, the order is put into a queue, other food is made in the meantime, my food is ready, and the server brings it to me.

A Non-AJAX Example: setTimeout()

console.log("Legen...");

setTimeout(() => {
  console.log("DARY!");
}, 2000);

console.log("Wait for it...");

setTimeout() is actually an asynchronous function, which executes its callback after waiting for the allotted time to expire.

Further Reading:

• The Evolution of Asynchronous JavaScript
• Fetch
• Promises
• Postman – good tool for testing out APIs
• GETs, POSTs and DELETEs, man! Intro to Postman - Instructor video for getting started with Postman