Stopping Function Execution Upon Event Trigger

In my recent project, I encountered a challenge where I needed to gracefully terminate an ongoing asynchronous process when a specific event occurred. Typically, when an error is thrown in any part of the process, the entire process halts as expected. However, the issue arose when I tried to stop the process from within an event listener; doing so didn’t interrupt the process flow as anticipated. Instead, it resulted in an “Unhandled Rejection” or “DOMException”. This blog post aims to explore and resolve the problem of terminating an asynchronous function effectively from an event listener while avoiding common pitfalls associated with error handling in asynchronous JavaScript environments.

The Core of the Problem

Imagine a function that performs a series of steps, each delayed by asynchronous waits. The requirement is that if an external event signals a stop, the function should immediately cease operation. However, since the event’s stop command is executed within the scope of a listener and not the main function, it does not affect the ongoing operation as one might expect.

Examples and Why They Failed

1. Direct Error Throwing in Event Listener: I initially tried to throw an error directly within the event listener. It seemed straightforward: on receiving a stop signal, throw an error to stop all further processes. Nevertheless, this error wasn’t caught by the awaiting function as it was out of its scope, leading to unhandled promises.

1. Using Proxy for Error Emission: In another attempt, I utilized a JavaScript Proxy to intercept changes and throw errors. However, similar to the first method, this error occurred out of the asynchronous function’s context and resulted in an unhandled exception rather than stopping the process.

1. Function Invocation to Throw Error: Even when abstracting the error-throwing into a separate function that gets called by the event listener, it didn’t solve the issue. The error was still thrown in the context of the event listener.

1. Promise Rejection on Event: Wrapping the whole function in a Promise and rejecting it on the event seemed promising. Yet, due to the nature of how promises work, the remaining part of the function execution didn’t stop immediately upon rejection, and hence the process continued until its next asynchronous checkpoint.

What Worked: Using AbortController

Given the complications with directly throwing errors or rejecting promises from the event listener, using AbortController emerged as a practical solution. Here’s why it works:

• Immediate Signal: AbortController provides a way to pass a signal to the asynchronous operations that can be checked at various stages of execution. If the abort condition is triggered, the operations can be immediately halted by throwing an error.

• Elegant and Manageable: This approach keeps the management of asynchronous operations tidy and allows various operations to listen for the same abort signal.

Example with Workable Solution

(async () => {
  const delay = (ms, signal) => {
    if (signal?.aborted) {
      throw new Error(signal.reason || 'Operation aborted');
    }
    return new Promise(resolve => setTimeout(resolve, ms));
  };

  const processEmitter = new EventEmitter();
  const controller = new AbortController();
  const { signal } = controller;

  async function startProcess(params) {
    try {
      processEmitter.on('stop', () => {
        controller.abort('External event requested stop');
      });

      await delay(2000, signal);
      console.log('Step 1 complete');
      await delay(2000, signal);
      console.log('Step 2 complete');
      // Additional steps follow
    } catch (err) {
      console.error('Process terminated:', err.message);
    }
  }

  startProcess();
  setTimeout(() => processEmitter.emit('stop'), 1000); // Stop before all steps complete
})();

Conclusion

Effectively terminating an asynchronous process in JavaScript, especially from external events, requires careful consideration regarding the execution context and error handling. While direct error throwing and promise rejection from event listeners are not effective due to their execution contexts, utilizing AbortController provides a robust way to preemptively terminate asynchronous operations based on external signals. This approach enhances the manageability and readability of the code, making it easier to maintain and debug.