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Fetch - Abort

Network Requests: Fetch - Abort

What does it mean to "abort a fetch"?​

View Answer:
Interview Response: Aborting a fetch means cancelling an ongoing fetch request. This can be useful when you no longer need the response or if the fetch takes too long.

Here is a simple code example:

const controller = new AbortController();
const signal = controller.signal;

// Start fetch
fetch('https://api.example.com/data', { signal })
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(err => {
    // On abort, the promise is rejected with an AbortError
    if (err.name === 'AbortError') {
      console.log('Fetch aborted');
    } else {
      console.error('Another error', err);
    }
  });

// Abort fetch after 2 seconds
setTimeout(() => controller.abort(), 2000);

What is the AbortController?​

View Answer:
Interview Response: AbortController is an interface that allows you to cancel one or more web requests as and when desired using an AbortSignal.

Code Example:

const controller = new AbortController();

How is an AbortSignal used?​

View Answer:
Interview Response: AbortSignal is used by passing it to the fetch request. When the associated AbortController’s abort() method is called, the fetch is cancelled.

Code Example:

// Initialize a new AbortController
const controller = new AbortController();

// Get the AbortSignal from the AbortController
const signal = controller.signal;

// Start a fetch request
fetch('https://api.example.com/data', { signal })
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(err => {
    if (err.name === 'AbortError') {
      console.log('Fetch operation was aborted');
    } else {
      console.error('Fetch operation failed', err);
    }
  });

// Abort the fetch request after 5 seconds
setTimeout(() => controller.abort(), 5000);

What happens when you call the abort() method on an AbortController?​

View Answer:
Interview Response: The abort() method signals to cancel the fetch operations associated with it. This causes the fetch promises to reject with an AbortError.

Code Example:

// Create a new AbortController
const controller = new AbortController();

// Fetch some data, passing the AbortSignal as part of the options
fetch('https://api.example.com/data', { signal: controller.signal })
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => {
    if (error.name === 'AbortError') {
      console.log('Fetch operation was aborted');
    } else {
      console.error('Fetch operation failed', error);
    }
  });

// After 5 seconds, call abort() on the controller
setTimeout(() => controller.abort(), 5000);

What error is thrown when a fetch is aborted?​

View Answer:
Interview Response: When a fetch is aborted, it rejects the promise with a DOMException named AbortError.

Code Example:

fetch('https://example.com', { signal })
  .then(response => {
})
  .catch(err => {
    if (err.name === 'AbortError') {
      console.log('AbortError: Fetch request aborted');
    }
});

How do you handle an abort error?​

View Answer:
Interview Response: You can handle an abort error in a catch block of the fetch promise, checking if the error's name is 'AbortError'.

Code Example:

fetch('https://example.com', { signal })
  .then(response => {
})
  .catch(err => {
    if (err.name === 'AbortError') {
      console.log('AbortError: Fetch request aborted');
    }
});

Can you reuse an AbortController after calling abort()?​

View Answer:
Interview Response: No, once the abort() method is called on an AbortController, it can't be reset or reused.

Can aborting fetches improve the performance of a web application?​

View Answer:
Interview Response: Yes, aborting fetches can potentially improve the performance of a web application. This can be especially useful in situations where you have a new fetch that supersedes an ongoing one.

Code Example:

let controller = new AbortController();

// Function to fetch data based on a query
function fetchData(query) {
  // If there's an ongoing fetch, abort it
  if (controller) controller.abort();

  // Create a new controller for the new fetch
  controller = new AbortController();

  // Start the new fetch
  fetch(`https://api.example.com/search?q=${query}`, { signal: controller.signal })
    .then(response => response.json())
    .then(data => console.log(data)) // Do something with the data here
    .catch(error => {
      if (error.name !== 'AbortError') {
        console.error('Fetch operation failed', error);
      }
    });
}

// Function to handle input from the user
function handleInput(input) {
  fetchData(input);
}

Does aborting a fetch request also abort the process on the server side?​

View Answer:
Interview Response: No, aborting a fetch request only stops the client from listening to the response. It doesn't affect the process on the server side, which will usually complete as normal.

How does the browser indicate an aborted fetch to the server?​

View Answer:
Interview Response: The browser doesn't signal the server when a fetch is aborted. Once the request is sent, it's processed server-side regardless of client-side cancellation.

Can you abort a fetch after the response has started to arrive?​

View Answer:
Interview Response: Yes, you can call `controller.abort()` after the response starts arriving. This will reject the fetch promise with an `AbortError`, stopping the processing of the response.

What happens to the response object of an aborted fetch?​

View Answer:
Interview Response: The response object of an aborted fetch is not fully received or processed. Attempting to read from it will reject the promise with an `AbortError`.

What if you don't provide an AbortSignal to a fetch request? What Happens?​

View Answer:
Interview Response: If you don't provide an AbortSignal to a fetch request, the fetch operation will continue as normal and cannot be programmatically cancelled using the AbortController mechanism.

Here's an example of a fetch request without an AbortSignal:

fetch('https://api.example.com/data')
  .then(response => {
    if (!response.ok) {
      throw new Error('Network response was not ok');
    }
    return response.json();
  })
  .then(data => console.log(data))
  .catch(error => console.error('There has been a problem with your fetch operation: ', error));

Can you abort a fetch request from a worker?​

View Answer:
Interview Response: Yes, you can abort a fetch request from a web worker. The AbortController and AbortSignal are transferrable objects and can be used in web workers.

Code Example:

Here's an example of how to abort a fetch request from a worker:

In your main JavaScript file, you can post the AbortSignal to the worker:

const controller = new AbortController();
const worker = new Worker('worker.js');

worker.postMessage({ signal: controller.signal }, [controller.signal]);

// Abort the fetch operation after 2 seconds
setTimeout(() => controller.abort(), 2000);

Then in worker.js, you handle the message and use the signal in a fetch request:

self.onmessage = (event) => {
  const { signal } = event.data;

  fetch('https://example.com', { signal })
    .then(response => response.json())
    .then(data => self.postMessage(data))
    .catch(err => {
      if (err.name === 'AbortError') {
        console.log('Fetch operation aborted');
      } else {
        console.error('Fetch operation failed:', err);
      }
    });
};

In this example, the main script creates an AbortController, passes the AbortSignal to the worker, and then aborts the fetch after 2 seconds. The worker listens for messages, receives the AbortSignal, uses it in a fetch operation, and then sends the fetched data back to the main script. If the fetch is aborted, it logs an appropriate message.

What is the default state of an AbortSignal's aborted flag?​

View Answer:
Interview Response: The aborted flag is initially set to false. It becomes true when abort() is called on the associated AbortController.

Code Example:

// Create a new AbortController
const controller = new AbortController();

// Get the signal from the controller
const signal = controller.signal;

// Log the initial state of the aborted flag
console.log(signal.aborted); // Outputs: false

controller.abort(); // abort controller

// Log the final state of the aborted flag
console.log(signal.aborted); // Outputs: true

Is abort functionality exclusive to fetch or does it work with other APIs too?​

View Answer:
Interview Response: Abort functionality is not exclusive to fetch. It can be used with any API that supports or is configured to work with the AbortSignal, including DOM APIs and various async tasks.

Technical Response: The AbortController and AbortSignal APIs are not exclusive to the Fetch API. They're part of the DOM standard and can be used to abort various kinds of operations, such as ongoing Fetch requests, streams, or even ongoing animations using the Web Animations API.

Here's a simple example of using `AbortController` with the `addEventListener` method:

// Create a new AbortController
const controller = new AbortController();

// Get the AbortSignal from the controller
const signal = controller.signal;

// Listen for click events on the document
document.addEventListener('click', () => {
  console.log('Document was clicked');
}, { signal });

// Call abort on the controller after 5 seconds
setTimeout(() => {
  controller.abort();
  console.log('No longer listening for clicks');
}, 5000);

In this code:

  • A new AbortController is created and its AbortSignal is obtained.
  • A click event listener is added to the document, with the AbortSignal passed in the options object. This associates the event listener with the abort controller.
  • A setTimeout call is set up to call controller.abort() after 5 seconds. This will remove the click event listener from the document, as the AbortSignal associated with it has been aborted.
  • After the abort call, "No longer listening for clicks" will be logged to the console, and no further click events on the document will be logged.

This is a simplistic example and there's not often a need to abort event listeners like this in practice, but it shows how the AbortController/AbortSignal APIs can be used outside of the Fetch API.

As always, be sure to check for compatibility as not all APIs or browsers may support AbortController and AbortSignal.

Is the AbortController part of JavaScript or the browser API?​

View Answer:
Interview Response: AbortController is part of the Web API provided by browsers, not part of the core JavaScript language. It's specified in the DOM Living standard.

Can you use AbortController with async/await syntax?​

View Answer:
Interview Response: Yes, AbortController can be used with async/await syntax. When you abort, it causes the awaited fetch to throw an 'AbortError'.

Code Example:

Here's how you can use AbortController with async/await:

// Create an instance of AbortController
const controller = new AbortController();
const signal = controller.signal;

async function fetchData() {
  try {
    const response = await fetch('https://example.com', { signal });
    const data = await response.json();
    console.log(data);
  } catch (err) {
    if (err.name === 'AbortError') {
      console.log('Fetch operation aborted');
    } else {
      console.error('Fetch operation failed:', err);
    }
  }
}

fetchData();

// After 2 seconds abort the fetch operation
setTimeout(() => controller.abort(), 2000);

In this example, the fetch operation is aborted after 2 seconds, causing the await fetch() line to throw an AbortError, which is then caught in the catch block.

Is it possible to cancel or abort an ongoing Fetch?​

View Answer:
Interview Response: Yes, it's possible to abort an ongoing Fetch using the AbortController and AbortSignal interfaces in the Fetch API. The `abort()` method on the AbortController cancels the Fetch.

Technical Response: Yes, there is a special built-in object for such purposes: AbortController. We can use it to abort, fetch, and do other asynchronous tasks. The usage is very straightforward. The AbortController interface represents a controller object that allows you to abort one or more Web requests as and when desired. You can create a new AbortController object using the AbortController.AbortController() constructor. Communicating with a DOM request is done using an AbortSignal object (calling abort()).

Code Example:

// Initialize a new AbortController
const controller = new AbortController();

// Get the AbortSignal from the AbortController
const signal = controller.signal;

// Start a fetch request
fetch('https://api.example.com/data', { signal })
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(err => {
    if (err.name === 'AbortError') {
      console.log('Fetch operation was aborted');
    } else {
      console.error('Fetch operation failed', err);
    }
  });

// Abort the fetch request after 5 seconds
setTimeout(() => controller.abort(), 5000);

Could you perhaps clarify the purpose of the AbortController Object?​

View Answer:
Interview Response: AbortController is a built-in JavaScript object that enables canceling ongoing activities, like Fetch requests, by invoking its `abort()` method, sending a signal to stop the operation.

Technical Details: We must create a new AbortController constructor to implement the AbortController object. The controller is an object with a single abort method and a property signal that allows us to put event listeners on it. When abort() gets called, the controller invokes. The abort event transmits by controller.signal, and the attribute "controller. signal. aborted" becomes true. When abort() invokes on it, AbortController passes abort events.

Code Example:

let controller = new AbortController();
let signal = controller.signal;

// The party that performs a cancelable operation
// gets the "signal" object
// and sets the listener to trigger when controller.abort() is called
signal.addEventListener('abort', () => console.log('abort!'));

// The other party, that cancels (at any point later):
controller.abort(); // abort!

// The event triggers and signal.aborted becomes true
console.log(signal.aborted); // true

Is it possible to cancel an event without using the AbortController?​

View Answer:
Interview Response: Yes, it's possible, but it typically involves setting and checking flags or using older APIs like XMLHttpRequest, which are less straightforward and flexible compared to using AbortController.

Code Example:

Here is a simplified example of how one might cancel a fetch request without AbortController:

let shouldAbort = false;

fetch('https://example.com')
    .then(response => {
        if (shouldAbort) throw new Error('Operation Aborted');
        return response.json();
    })
    .then(data => {
        if (shouldAbort) throw new Error('Operation Aborted');
        console.log(data);
    })
    .catch(err => console.error(err));

// Somewhere else in your code where you want to abort the fetch
shouldAbort = true;

In this case, we're using a flag (shouldAbort) to indicate whether the fetch should be cancelled. Note that this won't actually stop the fetch request from executing, it only prevents your code from handling the response. It's less effective than using AbortController.

Why is it beneficial to use the AbortController object for cancelling fetch operations instead of implementing independent event listening in our code?​

View Answer:
Interview Response: AbortController simplifies cancellation of fetch operations by creating a standardized, reusable solution, reducing code complexity, and making error handling and abort scenarios more predictable and easier to debug.

Code Example:

Here's a simple code example showing how to use AbortController for cancelling fetch operations:

// Create an instance of AbortController
let controller = new AbortController();
let signal = controller.signal;

// Start the fetch operation
fetch('https://example.com', { signal })
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(err => {
        if (err.name === 'AbortError') {
            console.log('Fetch operation aborted');
        } else {
            console.error('Fetch operation failed:', err);
        }
    });

// Somewhere else in your code where you want to abort the fetch
controller.abort();

In this example, if you call controller.abort(), it will cancel the fetch operation, causing the promise to be rejected with an AbortError.

Can you cancel an ongoing fetch with the AbortController?​

View Answer:
Interview Response: Yes, to be able to cancel fetch, we must pass the signal property of an AbortController as a fetch option. The fetch method knows how to work with AbortController. It listens to abort events on a signal. Now, to abort, we call controller.abort(). At that point, fetch extracts the event from the signal and aborts the request.

Code Example:

Here is a practical example of canceling an ongoing fetch operation with AbortController:

// Create an instance of AbortController
const controller = new AbortController();
const signal = controller.signal;

// Start fetch operation
fetch('https://example.com', { signal })
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => {
        if (error.name === 'AbortError') {
            console.log('Fetch operation aborted');
        } else {
            console.error('Other error', error);
        }
    });

// After 2 seconds abort the fetch operation
setTimeout(() => controller.abort(), 2000);

In this example, the fetch operation is aborted after 2 seconds, cancelling the ongoing fetch operation. If the fetch operation is still ongoing after 2 seconds, an AbortError will be thrown.

Can the AbortController cancel or abort multiple fetches at once?​

View Answer:
Interview Response: Yes, AbortController is scalable by default. It allows us to cancel multiple fetches at once, which can be exceptionally helpful when dealing with an array.

Code Example:

let urls = [...]; // a list of urls to fetch in parallel

let controller = new AbortController();

// an array of fetch promises
let fetchJobs = urls.map(url => fetch(url, {
  signal: controller.signal
}));

let results = await Promise.all(fetchJobs);

// if controller.abort() is called from anywhere,
// it aborts all fetches