Understanding Asynchronous Recursive Function Calls in JavaScript

Adam C. | 3 weeks ago

Asynchronous recursive function calls are a powerful technique used in JavaScript for handling tasks that require repeated asynchronous operations until a certain condition is met. In this blog post, we'll explore this concept using a real-world example and discuss how to ensure that the final result is correctly propagated back to the initial caller.

Background

Imagine a scenario where you need to fetch data from an API in an asynchronous manner, but the data retrieval process may take some time. You want to check the status of the data fetch operation periodically until the data is fully retrieved.

Example Scenario

Let's consider a simplified example where we have an asynchronous function fetchStatus() that fetches the status of a job from an API. We want to repeatedly check the status until the job is marked as "COMPLETED."

// Recursive function to check status
const checkStatus = async () => {
  // Simulate fetching status from an API
  const getResponse = await fetchStatus(); // Assume fetchStatus() fetches the status from an API

  if (getResponse.JobStatus === "COMPLETED") {
    // If job is completed, resolve with the result
    return getResponse.Result;
  }

  if (getResponse.JobStatus === "PROCESSING") {
    // If job is still processing, wait for 1 second and call checkStatus again
    await new Promise((resolve) => setTimeout(resolve, 1000));
    return checkStatus(); // Recursive call
    // OR:
    // return new Promise((resolve) => {
    //     setTimeout(() => resolve(checkStatus()), 1000);
    //});
  }

  // Handle other job statuses here if needed
  throw new Error("Unexpected job status");
};

// Initial call to checkStatus
const getResult = async () => {
  try {
    const result = await checkStatus();
    console.log("Final result:", result);
    return result; // Result is bubbled up to the initial caller
  } catch (error) {
    console.error("Error checking status:", error);
    throw error; // Error is also bubbled up if encountered
  }
};

// Usage example
getResult()
  .then((result) => {
    console.log("Success:", result);
  })
  .catch((error) => {
    console.error("Error:", error);
  });

In this example:

1. We define the checkStatus function that asynchronously fetches the job status and handles different status scenarios.
2. Inside checkStatus, if the job status is "PROCESSING," we wait for 1 second using setTimeout and then recursively call checkStatus again.
3. The getResult function initiates the process by calling checkStatus, and the final result or error is bubbled up to the initial caller through promises.

With Callbacks

To use callbacks in the example of asynchronous recursive function calls, you can modify the code to pass a callback function as a parameter to the checkStatus function. Here's how you can do it:

// Recursive function to check status with callback
const checkStatus = async (callback) => {
  // Simulate fetching status from an API
  const getResponse = await fetchStatus(); // Assume fetchStatus() fetches the status from an API

  if (getResponse.JobStatus === "COMPLETED") {
    // If job is completed, invoke the callback with the result
    callback(null, getResponse.Result);
    return;
  }

  if (getResponse.JobStatus === "PROCESSING") {
    // If job is still processing, wait for 1 second and call checkStatus again
    await new Promise((resolve) => setTimeout(resolve, 1000));
    checkStatus(callback); // Recursive call with the same callback
    return;
  }

  // Handle other job statuses here if needed
  callback(new Error("Unexpected job status"));
};

// Usage example with callback
const handleStatus = (error, result) => {
  if (error) {
    console.error("Error checking status:", error);
    return;
  }
  console.log("Final result:", result);
};

// Initiate the process by calling checkStatus with the callback
checkStatus(handleStatus);

In this updated code:

• The checkStatus function now accepts a callback function as a parameter.
• When the job status is "COMPLETED," it invokes the callback with the result.
• When the job status is "PROCESSING," it recursively calls checkStatus with the same callback.
• The handleStatus function serves as the callback to process the final result or error.

This approach allows you to handle the asynchronous recursive calls using callbacks, providing more flexibility and control over the flow of the asynchronous operations.