JavaScript’s Fetch API is a powerful tool for making network requests. It’s relatively straightforward, but there are some common pitfalls developers can encounter. This guide highlights these mistakes and provides tips to avoid them, ensuring your fetch calls are efficient and error-free.

Mistake 1: Not Handling Errors Properly

The Issue:

Many developers assume that a successful fetch request (i.e., one that doesn’t throw a network error) means the response is valid. However, fetch will only throw an error for network issues, not for HTTP errors like 404 or 500.

How to Avoid:

Always check the response status before processing the response data.

fetch('https://api.example.com/data')
  .then(response => {
    if (!response.ok) {
      throw new Error(`HTTP error! Status: ${response.status}`);
    }
    return response.json();
  })
  .then(data => console.log(data))
  .catch(error => console.error('Fetch error:', error));

Mistake 2: Ignoring CORS Issues

The Issue:

Cross-Origin Resource Sharing (CORS) errors can be perplexing. These errors occur when a web page tries to fetch resources from a different domain without the proper permissions.

How to Avoid:

Ensure the server you’re requesting from includes the correct CORS headers. For example, the server should send headers like:

Access-Control-Allow-Origin: *

On the client side, you can specify credentials if needed:

fetch('https://api.example.com/data', {
  credentials: 'include' // or 'same-origin'
})
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('CORS error:', error));

Mistake 3: Not Using Async/Await

The Issue:

Using .then() chains can quickly become unreadable and hard to manage, especially with nested calls.

How to Avoid:

Use async/await to handle asynchronous fetch calls more cleanly.

async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    if (!response.ok) {
      throw new Error(`HTTP error! Status: ${response.status}`);
    }
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error('Fetch error:', error);
  }
}

fetchData();

Mistake 4: Forgetting to Handle JSON Parse Errors

The Issue:

Assuming that the response is always valid JSON can lead to unhandled exceptions when the JSON is malformed or the response isn’t JSON at all.

How to Avoid:

Wrap your JSON parsing in a try/catch block.

async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    if (!response.ok) {
      throw new Error(`HTTP error! Status: ${response.status}`);
    }
    const data = await response.json();
    console.log(data);
  } catch (error) {
    if (error.name === 'SyntaxError') {
      console.error('Error parsing JSON:', error);
    } else {
      console.error('Fetch error:', error);
    }
  }
}

fetchData();

Mistake 5: Not Setting Headers Correctly

The Issue:

Some APIs require specific headers (e.g., Content-Type, Authorization) that developers might forget to include.

How to Avoid:

Always set the necessary headers in your fetch request.

fetch('https://api.example.com/data', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Authorization': 'Bearer YOUR_TOKEN_HERE'
  },
  body: JSON.stringify({ key: 'value' })
})
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Fetch error:', error));

Mistake 6: Not Handling Large Responses Efficiently

The Issue:

Fetching and parsing large responses can be slow and resource-intensive.

How to Avoid:

Use streams to process large responses incrementally.

fetch('https://api.example.com/large-data')
  .then(response => {
    const reader = response.body.getReader();
    const decoder = new TextDecoder();
    return reader.read().then(function processText({ done, value }) {
      if (done) {
        return;
      }
      console.log(decoder.decode(value));
      return reader.read().then(processText);
    });
  })
  .catch(error => console.error('Fetch error:', error));

By being mindful of these common mistakes and implementing the suggested solutions, you can make your fetch calls more robust and reliable. Happy coding!