Explanation for “uncaught (in promise) error”

The “uncaught (in promise) error” occurs when a JavaScript promise rejects but no error handler was attached to handle the rejection. This error message is raised to inform you that a promise rejection went unhandled, and if not captured, it could potentially cause issues in your application.

In the specific case you mentioned, the error message is slightly different: “a listener indicated an asynchronous response by returning true, but the message channel closed before a response was received”. This error commonly occurs when working with asynchronous messaging systems or APIs.

Let’s break down the error message:

• “a listener indicated an asynchronous response by returning true”: This indicates that you have implemented a listener or callback function that returns “true” to indicate it will handle an asynchronous response.
• “but the message channel closed before a response was received”: This means that the communication channel through which the response was expected to be received was closed before the response actually arrived.

Example

    
// Assume you have an asynchronous function that returns a promise
function fetchData() {
  return new Promise((resolve, reject) => {
    // Imagine the fetchData function is making an API call
    // and expects a response in the callback
    const response = apiCall(...);
    
    // If the response is received successfully, resolve the promise
    response.on('success', (data) => {
      resolve(data);
    });
    
    // In this case, let's imagine a condition where the listener returns true but response channel closed prematurely
    if (prematureCondition) {
      response.close(); // Simulating closing the response channel
      return true; // Returning true to indicate handling asynchronous response
    }
    
    // If some error occurs, reject the promise
    response.on('error', (error) => {
      reject(error);
    });
  });
}

// Now, if we use this fetchData function and do not handle the promise rejection,
// we will encounter the "uncaught (in promise) error"
fetchData()
  .then((data) => {
    // Handle the resolved promise and use the data
  })
  .catch((error) => {
    // Handle the rejected promise and display the error message
    console.error("Error occurred while fetching data:", error);
  });
    
  

In the above example, we have a function called “fetchData” that returns a promise. It makes an API call and expects a response through an event-based asynchronous system. In a particular condition, we simulate a scenario where the listener conditionally returns true and prematurely closes the response channel. As a consequence, the promise is rejected without any error handler attached, which triggers the “uncaught (in promise) error” message.

To resolve this error, make sure to attach a catch block to handle promise rejections properly. By adding a catch block, you can handle any rejected promises, log useful error messages, and prevent unhandled promise rejections from causing issues in your application.