Master the fetch API CORS like a pro with these real-life coding examples

Table of content

  1. Introduction
  2. Understanding the Fetch API
  3. CORS – Cross-Origin Resource Sharing
  4. Real-life Coding Example #1: Fetching data from external API
  5. Real-life Coding Example #2: Uploading files with CORS
  6. Real-life Coding Example #3: PUT request with CORS
  7. Real-life Coding Example #4: Handling errors with CORS
  8. Conclusion


Have you ever struggled with the CORS policy while trying to fetch data from a remote server using the JavaScript fetch API? If you have, then fear not! In this article, we will take a deep dive into the fetch API CORS and explore how it works with real-life coding examples.

Cross-Origin Resource Sharing (CORS) is a security feature implemented in modern browsers that restricts web pages from making requests to a different domain. This policy affects the fetch API greatly since it sends requests to a different domain from the one the webpage originates from. As a result, requests sent from the browser can be blocked by the server due to the CORS policy.

In this article, we will explore different ways to handle CORS issues while fetching data from remote servers using the fetch API. We will examine how to set up a CORS proxy server to bypass CORS policy errors and how to configure CORS on the server-side to allow access to certain domains.

With these real-life coding examples, you will master the fetch API CORS like a pro, and you will be able to fetch data from any remote server without fear of CORS errors. So grab your favorite code editor and let's get started on this adventure!

Understanding the Fetch API

The Fetch API is a powerful tool for making network requests in JavaScript, allowing developers to easily interact with web services and fetch data from remote sources. To use the Fetch API, you'll need to understand how to construct requests and handle responses.

At its core, the Fetch API relies on a set of functions that allow you to send HTTP requests and receive responses. The most commonly used functions are fetch(), which sends a request and returns a promise, and then(), which handles the response once it arrives.

To use the Fetch API, you'll need to first construct a request object that specifies the URL you want to fetch. You can add additional configuration options to the request, such as headers or request methods, as needed.

Once you've constructed the request, you can send it using the fetch() function. This will return a promise that resolves to a response object, which you can then use to retrieve the data returned from the server.

It's important to note that the Fetch API obeys the same-origin policy, which restricts requests to resources with the same origin as the requesting script. To make cross-origin requests, you may need to use CORS or other workarounds.

To master the Fetch API, it's important to practice and experiment with different requests and responses. Try fetching data from different sources, and test out different configuration options to see how they affect the request and response. With practice, you'll quickly become a pro at using the Fetch API to interact with the web.

CORS – Cross-Origin Resource Sharing

Cross-Origin Resource Sharing (CORS) is an important concept to understand when working with the fetch API. Simply put, CORS is a security feature implemented by web browsers that restricts a web page from calling resources from a different domain. This can be problematic when making API requests to a server that is not hosted on the same domain as your web page.

To enable CORS, server-side developers need to set the appropriate headers in the response, allowing the client to access resources from other domains. If the server does not allow this, then the browser will block the request, and you will see a CORS error in the console.

Thankfully, the fetch API includes built-in CORS support, making it easy to handle these issues in your code. You can set the "mode" option to "cors," which will use the browser's built-in CORS implementation to make the request. This will ensure that the proper headers are set and allow you to fetch data from external domains.

In conclusion, understanding CORS is essential when working with APIs and the fetch API. By using the "mode" option with the fetch API and configuring the server-side headers correctly, you can develop applications that securely access external resources.

Real-life Coding Example #1: Fetching data from external API

To be able to master the Fetch API CORS, one must have a good understanding of how to fetch data from an external API. Here's a real-life coding example to help you get started!

Let's say you want to fetch data from the OpenWeatherMap API to display the current weather in a particular city. First, you need to get an API key from their website, which is free of charge for a limited number of API calls per day.

Once you have your API key, you can use the Fetch API to make a GET request to the API endpoint, passing in the necessary parameters such as the city name and the API key.

Here's an example of how to do it in JavaScript:

const apiKey = "your_api_key";
const cityName = "London";

  .then(response => response.json())
  .then(data => {
    // Do something with the data (e.g. display the temperature)
  .catch(error => {

The fetch function returns a Promise, which resolves with the response from the server. In this example, we convert the response into JSON format using the response.json() method.

Once we have the data, we can manipulate it and display it on the page as we wish. However, it's worth noting that APIs often have limitations on how often you can make requests, so make sure to read the API documentation thoroughly and handle errors gracefully.

Overall, fetching data from an external API is a useful skill to have when working with web development. With practice and experimentation, you can become proficient in using the Fetch API CORS and integrating external data into your projects.

Real-life Coding Example #2: Uploading files with CORS

Uploading files can be a challenging task, especially when CORS is involved. But fear not, with the Fetch API and our newfound knowledge of CORS, we can easily handle file uploads with a few lines of code. Here's an example of how to upload a file using the Fetch API and CORS.

  1. First, we need to create an HTML form that will allow the user to select the file they want to upload.
  <input type="file" id="file-input">
  <button type="button" onclick="uploadFile()">Upload</button>
  1. Next, we need to write the uploadFile function that will use the Fetch API to send the file to the server.
function uploadFile() {
  const fileInput = document.getElementById('file-input');
  const file = fileInput.files[0];
  const formData = new FormData();
  formData.append('file', file);

  fetch('', {
    method: 'POST',
    body: formData,
    mode: 'cors',
    credentials: 'include'
  .then(response => {
    console.log('File uploaded successfully!');
  .catch(error => {
    console.error('Error uploading file:', error);
  1. Finally, we need to ensure that the server is configured to accept CORS requests and handle file uploads. This will depend on the specific server-side technology you're using, but generally, you'll need to set the Access-Control-Allow-Origin header to allow requests from your domain, and handle the file upload using a middleware library like Express.

With these three steps, you should now be able to upload files with CORS using the Fetch API. As always, make sure to test thoroughly and handle errors gracefully to ensure a good user experience.

Real-life Coding Example #3: PUT request with CORS

To demonstrate a PUT request with CORS, let's first define what a PUT request is. A PUT request is an HTTP request method that updates an existing resource or creates a new resource if it is not found.

In this real-life coding example, we will make a PUT request to update an existing data record in the server, with support for CORS.

First, make sure that the CORS policy is correctly set up on the server-side to accept PUT requests from your client-side domain. Next, create a new XMLHttpRequest() object and set its method to PUT.

const xhr = new XMLHttpRequest();'PUT', '');

In the example above, we are sending a PUT request to update the data record with the ID of 1 on the API endpoint.

To send data with our PUT request, we set the Content-Type request header to application/json and pass our data as a JSON string through the send() method.

const data = { name: 'New Name', age: 25 };
xhr.setRequestHeader('Content-Type', 'application/json');

In this example, we are updating the name and age fields of the data record with our own values.

Finally, we handle the response with an event listener to see if the update was successful.

xhr.onreadystatechange = () => {
  if (xhr.readyState === 4) {
    if (xhr.status === 200) {
      console.log('Data record updated successfully!');
    } else {
      console.log('Error:', xhr.statusText);

In this example, we log a success message if the server returns a status code of 200, otherwise, we log the reason of the error with the statusText property.

That's it! You now have a basic understanding of how to make a PUT request with CORS. Remember, always ensure that the CORS policy is set up correctly on the server-side, and don't forget to handle errors effectively.

Real-life Coding Example #4: Handling errors with CORS

Handling errors is an important part of any web development project. When working with APIs that use CORS, it's essential to know how to handle errors that may arise due to the restriction policies in place. Here's a real-life coding example that demonstrates how to handle errors with CORS using the fetch API.

fetch('', {
  mode: 'cors'
  .then(response => {
    // handle successful response
  .catch(error => {
    // handle error response

In this example, we are using the fetch function to make a cross-origin request to We have set the mode option to 'cors' to ensure that CORS restrictions are applied to the request.

If the request succeeds, we can handle the response data in the then block. However, if an error occurs due to CORS restrictions, we can catch it in the catch block and handle it accordingly. In this case, we are simply logging the error to the console, but you can choose to handle it in a way that makes sense for your project.

It's important to note that when handling errors with CORS, the error message may not always provide a clear indication of what went wrong. In some cases, the browser may simply throw a generic error message without any detailed information. In these cases, it's up to you to debug the issue and find a solution.

To avoid CORS errors altogether, make sure that the APIs you are using have proper CORS headers set up. You should also ensure that your own server is configured correctly to allow cross-origin requests. By following best practices for CORS, you can minimize the risk of errors and ensure that your web applications are secure and reliable.


In , mastering the fetch API CORS can seem daunting at first, but with the right guidance and practice, it can become a valuable skill in your coding arsenal. The coding examples provided in this article should give you a solid foundation to build upon and experiment with. Remember to take the time to understand the concepts and don't be afraid to refer back to the documentation as needed.

It's also important to keep in mind that CORS can be a complex topic, and there may be cases where it requires additional research and troubleshooting. However, with persistence and patience, you'll be able to navigate any challenges that come your way.

As you continue to work with the fetch API CORS, don't forget to stay up-to-date with new developments in the field. Follow blogs, social media accounts, and attend events to keep your skills sharp and current. Finally, don't be afraid to seek out help from fellow developers and online communities. The programming world is full of supportive individuals who are willing to lend a hand and share their knowledge. Happy coding!

My passion for coding started with my very first program in Java. The feeling of manipulating code to produce a desired output ignited a deep love for using software to solve practical problems. For me, software engineering is like solving a puzzle, and I am fully engaged in the process. As a Senior Software Engineer at PayPal, I am dedicated to soaking up as much knowledge and experience as possible in order to perfect my craft. I am constantly seeking to improve my skills and to stay up-to-date with the latest trends and technologies in the field. I have experience working with a diverse range of programming languages, including Ruby on Rails, Java, Python, Spark, Scala, Javascript, and Typescript. Despite my broad experience, I know there is always more to learn, more problems to solve, and more to build. I am eagerly looking forward to the next challenge and am committed to using my skills to create impactful solutions.

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Posts

Begin typing your search term above and press enter to search. Press ESC to cancel.

Back To Top