Monitor fetch() API calls and responses in JavaScript

Question

Monitor fetch() API calls and responses in JavaScript

I’m looking to intercept fetch API requests and responses using JavaScript.

Specifically, I want to be able to capture the request URL before it is sent and also intercept the response once it has been received.

The code below demonstrates how to intercept responses for all instances of XMLHTTPRequest.

(function(open) {
  XMLHttpRequest.prototype.open = function(XMLHttpRequest) {
    var self = this;
    this.addEventListener("readystatechange", function() {
      if (this.responseText.length > 0 &&
          this.readyState == 4 &&
          this.responseURL.indexOf('www.google.com') >= 0) {

        Object.defineProperty(self, 'response', {
          get: function() { return bValue; },
          set: function(newValue) { bValue = newValue; },
          enumerable: true,
          configurable: true
        });
        self.response = 'updated value' // Intercepted Value 
      }
    }, false);
    open.apply(this, arguments);
  };
})(XMLHttpRequest.prototype.open);

Now, I am interested in implementing a similar feature for the fetch() API. How can I achieve this?

javascript ajax xmlhttprequest interceptor fetch-api

Answer 1

Answer №1

Previous solutions have outlined the basic structure for mocking fetch in the browser, but some key details have been left out.

The main answer provides a general approach to replacing the window.fetch function with a custom implementation that intercepts the call and forwards the arguments to fetch. However, this method does not allow the interception function to manipulate the response (e.g., access status or body contents), making it limited to just logging request parameters.

Another solution introduces an async function to enable the interceptor to use await on the fetch promise and potentially interact with the response data. But there are issues such as unnecessary closures, lack of guidance on reading the response body without altering it, and a variable aliasing bug causing a stack overflow.

This alternative answer is more comprehensive but includes unrelated content in the callback and overlooks crucial information on cloning the response to safely collect the body using the interceptor. It also fails to demonstrate how a mock response could be implemented.

Below is a refined example addressing these concerns, showcasing parameter logging, non-destructive body retrieval through response cloning (using clone()), and the option to provide a mock response if needed.

const {fetch: origFetch} = window;
window.fetch = async (...args) => {
  console.log("fetch called with args:", args);
  const response = await origFetch(...args);

  /* work with the cloned response in a separate promise
     chain -- could use the same chain with `await`. */
  response
    .clone()
    .json()
    .then(data => console.log("intercepted response data:", data))
    .catch(err => console.error(err));

  /* the original response can be resolved unmodified: */
  //return response;

  /* or mock the response: */
  return new Response(JSON.stringify({
    userId: 1,
    id: 1,
    title: "Mocked!!",
    completed: false
  }));
};

// test it out with a typical fetch call
fetch("https://jsonplaceholder.typicode.com/todos/1")
  .then(response => response.json())
  .then(data => console.log("original caller received:", data))
  .catch(err => console.error(err));

Answer 2

Previous solutions have outlined the basic structure for mocking fetch in the browser, but some key details have been left out.

The main answer provides a general approach to replacing the window.fetch function with a custom implementation that intercepts the call and forwards the arguments to fetch. However, this method does not allow the interception function to manipulate the response (e.g., access status or body contents), making it limited to just logging request parameters.

Another solution introduces an async function to enable the interceptor to use await on the fetch promise and potentially interact with the response data. But there are issues such as unnecessary closures, lack of guidance on reading the response body without altering it, and a variable aliasing bug causing a stack overflow.

This alternative answer is more comprehensive but includes unrelated content in the callback and overlooks crucial information on cloning the response to safely collect the body using the interceptor. It also fails to demonstrate how a mock response could be implemented.

Below is a refined example addressing these concerns, showcasing parameter logging, non-destructive body retrieval through response cloning (using clone()), and the option to provide a mock response if needed.

const {fetch: origFetch} = window;
window.fetch = async (...args) => {
  console.log("fetch called with args:", args);
  const response = await origFetch(...args);

  /* work with the cloned response in a separate promise
     chain -- could use the same chain with `await`. */
  response
    .clone()
    .json()
    .then(data => console.log("intercepted response data:", data))
    .catch(err => console.error(err));

  /* the original response can be resolved unmodified: */
  //return response;

  /* or mock the response: */
  return new Response(JSON.stringify({
    userId: 1,
    id: 1,
    title: "Mocked!!",
    completed: false
  }));
};

// test it out with a typical fetch call
fetch("https://jsonplaceholder.typicode.com/todos/1")
  .then(response => response.json())
  .then(data => console.log("original caller received:", data))
  .catch(err => console.error(err));

Answer 3

Answer №2

If you need to intercept the fetch request and parameters, one way to do it is shown below. This method worked for me in resolving my issue.

 const constantMock = window.fetch;
 window.fetch = function() {
     // Extract the parameters from the arguments
     // Intercept the parameters here 
    return constantMock.apply(this, arguments)
 }

Answer 4

If you need to intercept the fetch request and parameters, one way to do it is shown below. This method worked for me in resolving my issue.

 const constantMock = window.fetch;
 window.fetch = function() {
     // Extract the parameters from the arguments
     // Intercept the parameters here 
    return constantMock.apply(this, arguments)
 }

Answer 5

Answer №3

To capture the response body, you must create a new Promise and handle it within the "then" code block. This approach worked for me and is suitable for actual applications like React.

const originalFetch = window.fetch;
window.fetch = function() {
  console.log(arguments);

  return new Promise((resolve, reject) => {
    originalFetch
      .apply(this, arguments)
      .then((response) => {
        if (response.url.indexOf("/me") > -1 && response.type !== "cors") {
          console.log(response);
          // Perform specific actions based on conditions
        }
        resolve(response);
      })
      .catch((error) => {
        reject(error);
      })
  });
}

Answer 6

To capture the response body, you must create a new Promise and handle it within the "then" code block. This approach worked for me and is suitable for actual applications like React.

const originalFetch = window.fetch;
window.fetch = function() {
  console.log(arguments);

  return new Promise((resolve, reject) => {
    originalFetch
      .apply(this, arguments)
      .then((response) => {
        if (response.url.indexOf("/me") > -1 && response.type !== "cors") {
          console.log(response);
          // Perform specific actions based on conditions
        }
        resolve(response);
      })
      .catch((error) => {
        reject(error);
      })
  });
}

Answer 7

Answer №4

let apiFetch = window.fetch;
window.fetch = (...args) => (async(args) => {
    var response = await apiFetch(...args);
    console.log(response); // handle response here
    return response;
})(args);

Answer 8

let apiFetch = window.fetch;
window.fetch = (...args) => (async(args) => {
    var response = await apiFetch(...args);
    console.log(response); // handle response here
    return response;
})(args);

Answer 9

Answer №5

In addition to Hariharan's solution, I implemented a method to manage spinner state in Redux before and after each API call.

import store from './../store';

// Implement an interceptor for all fetch API requests
// Increases the redux spinner state when an API call is made
// Decreases the redux spinner state once the API call is resolved
(function() {
    const originalFetch = window.fetch;
    window.fetch = function() {
        store.dispatch({type: 'show-spinner'})
        return originalFetch.apply(this, arguments)
            .then((res) => {
                store.dispatch({type: 'hide-spinner'})
                return res;
            })
    }
})();

Answer 10

In addition to Hariharan's solution, I implemented a method to manage spinner state in Redux before and after each API call.

import store from './../store';

// Implement an interceptor for all fetch API requests
// Increases the redux spinner state when an API call is made
// Decreases the redux spinner state once the API call is resolved
(function() {
    const originalFetch = window.fetch;
    window.fetch = function() {
        store.dispatch({type: 'show-spinner'})
        return originalFetch.apply(this, arguments)
            .then((res) => {
                store.dispatch({type: 'hide-spinner'})
                return res;
            })
    }
})();

Monitor fetch() API calls and responses in JavaScript

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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