In building web apps with JavaScript, we may have to send a network request to a server and load new information when needed.
For instance, we can use a network request to

Submit an order
Load information, receive updates from a server, etc.

There are numerous ways to send a request and receive information from a server. A recommended approach is to use the Fetch API, which permit developers to make http request to web servers for resources needed to build web applications.

In this tutorial, you will learn about the JavaScript Fetch API. By the end of the tutorial, you should be able to use the fetch() method to fetch for information from a server. Let's get started !

What is the JavaScript Fetch API ?

The JavaScript Fetch API is a modern interface use to fetch for resources from a server. This is achieved by sending an HTTP request to the server from the web browser.

The Fetch API includes a fetch() method, which starts the process of fetching information from any web server, and working with the information fetched.
Using the fetch() method, we can receive data from a server, and even post data to the server.

What is the `fetch()` method ?

In JavaScript, the fetch() method is used to make asynchronous requests to the server and load the information that is returned by the server onto the web pages.

The syntax for the fetch() method is

fetch(URL, options)

The fetch() method accepts two arguments:

URL: The URL to access
- options: An object which consists of additional properties (methods, headers) sent to the server.

Without options, we will only be downloading contents of the URL

Fetching for resources

To fetch for some resource, you will call the fetch() method, and pass it one argument : aURL (which indicates the path to the resource you want to fetch).

const promise = fetch(url)

The fetch() method returns a Promise, which can either be resolved or rejected.

The promise resolves into a Response object. The Response object has a number of useful properties and methods to explore what we got from the request.
The promise rejects if the fetch() method was unable to make the HTTP-request. Rejections can be due to network problems, or if no such URL exits.

We chain the fetch() method with a then() and catch() method to enable us to use the response.

The then() method is basically used to handle asynchronous tasks (tasks that are time-consuming), such as fetching data from an API. When we get a response from the fetch() method, we will handle the response in the then().
Usually, we get a Response object from the fetch request. This object contains useful properties and methods.
If there is an error in fetching the request, for instance due to network problems or invalid URL, the catch() method will handle this error.

The complete syntax for fetching information from a server is as below:

fetch(url)
    .then(response => {
        // handle the response object
    })
    .catch(error => {
        // handle the error
    });

Reading the `Response` object

The Response object contains further data that needs to be converted into the required format in order to work with it.
There are several methods you can use to read the body of the response object, one such method is the json() method.

The json() method returns another Promise that decides the complete data of the fetched resource.
We then access the data in another .then() method

Example of fetching and reading data from an API

In the code below, we will fetch a dummy phone product from the dummyjson.com server, and then log the fetched data.

fetch('https://dummyjson.com/products/1')
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(err => console.error(err));

Steps for fetching and reading data from an API

Pass the fetch() method the required url to get the data from. In this instance, the URL will be https://dummyjson.com/products/1
Chain the fetch() method with the then() method
If all goes well, in the then() method, we will receive a Response object from the server
Extract the JSON body content from the Response object using the json() method.
The json() method will return a secondPromise. This Promise will resolve to contain the fetched data
- Pass this data to another then() method.
In the then()method, we can now print the fetched data to the console or assign it to a variable.
If there is an error in fetching the data, the catch() method will capture the error, and we can log that error

Example 2: Fetching data from fake store API

In this instance, we will fetch a single ecommerce product from the Fake Store API.
The fake Store API is a free online REST API we will be using to fetch pseudo-real data for our ecommerce website.

Call the fetch() method and pass the url or API endpoint as the argument.
The API endpoint or the url for this instance is : 'https://fakestoreapi.com/products/1'. It provides access to a single product

fetch('https://fakestoreapi.com/products/1')

Calling thisfetch() method returns a special JavaScript object called Promise. The Promise produces a value (Response object ) after an asynchronous operation successfully completes, or an error if unsuccessful.
Chain the fetch() method with athen() method to investigate the Response object we got from the API call.
The Response object is assigned to a response variable. (You can give the Response object any variable name of choice, res or response is the convention)
Use the json() method to view the complete content of the fetched resource ( For instance res.json() or response.json() depending on how you named your variable)

fetch('https://fakestoreapi.com/products/1').then(response => response.json())

Finally, access the fetched data by chaining another .then() method

fetch('https://fakestoreapi.com/products/1')
  .then(
  // view the  complete content of the response
  response => response.json())
  .then(
  //access the actual data 
  data => console.log(data))

The output will be an object containing some dummy phone data- title, price, description etc.

{
  "id": 1,
  "title": "Fjallraven - Foldsack No. 1 Backpack, Fits 15 Laptops",
  "price": 109.95,
  "description": "Your perfect pack for everyday use and walks in the forest. Stash your laptop (up to 15 inches) in the padded sleeve, your everyday",
  "category": "men's clothing",
  "image": "https://fakestoreapi.com/img/81fPKd-2AYL._AC_SL1500_.jpg",
  "rating": {
    "rate": 3.9,
    "count": 120
  }
}

Instead of logging the output, you can assign this value to declared variable, and access the relevant data using the appropriate key:value pair.

Handling the status code of the response

In the Response object, you can use the status and statusText properties to access the status code and status text, respectively.

An HTTP status code is a server response to a browser's request. When you visit a website, your browser sends a request to the site's server, and the server then responds to the browser's request with a three-digit code: the HTTP status code.

In the code below, we read the status code and status text from the server's response:

fetch('https://dummyjson.com/products/1')
    .then(response => {
  console.log(response.status) // check the status code of the response object
  console.log(response.statusText) // an interpretation of the code 
  return response.json() // read the actual data from body of the response object
})
    .then(data => console.log(data))
    .catch(err => console.error(err));

A successful request will give a status code of 200 and a status text of OK
If the requested URL throws a server error, the status code will be 500.
If the requested resource does not exist, the status code will be 400 and status text will be Not found

Summary

The Fetch API allows you to reqeust for a resource from a web server
The Fetch API provides access to the fetch() method that returns a promise which resolves into a Response object
Use the json() method to fetch the actual data
Use the status and statusText properties of theResponse object to investigate the status of the response
Use the catch() method to handle a failed request, for instance network problemsn

That's all devs, go ahead and try fetching some data from your favorite APIs.

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How to Fetch data from an API using the JavaScript Fetch method