How to convert JavaScript Callback APIs to Promises

Converting JavaScript Callback APIs to Promises can make your code more readable and easier to work with, especially when dealing with asynchronous operations. Here’s a step-by-step guide on how to convert callback-based APIs to use Promises:

1. Understand the Callback API#

Let’s start with a basic example of a callback-based API. Assume you have a function that performs an asynchronous operation and uses a callback to handle the result:

function fetchData(callback) {
    setTimeout(() => {
        const data = "Some data";
        callback(null, data); // First argument is error, second is result
    }, 1000);
}

2. Create a Promise Wrapper#

To convert this callback-based API to use Promises, you can create a new function that returns a Promise. In the Promise constructor, you will perform the asynchronous operation and call the resolve and reject methods to handle the result or error.

Here’s how you can wrap fetchData in a Promise:

function fetchDataPromise() {
    return new Promise((resolve, reject) => {
        fetchData((error, data) => {
            if (error) {
                reject(error); // Reject the Promise with an error
            } else {
                resolve(data); // Resolve the Promise with the result
            }
        });
    });
}

3. Use the Promisified Function#

Now that you have a Promise-based version of your function, you can use it with .then() and .catch() for handling results and errors:

fetchDataPromise()
    .then((data) => {
        console.log("Data received:", data);
    })
    .catch((error) => {
        console.error("Error occurred:", error);
    });

4. Handling Multiple Callbacks#

If the callback-based function involves multiple callbacks or has a different signature, you can adapt the Promise-based wrapper accordingly. For example:

Multiple Callbacks#

If the API has multiple callbacks, you can wrap them in a single Promise:

function complexOperation(callback1, callback2) {
    setTimeout(() => {
        callback1("Result from callback1");
        callback2("Result from callback2");
    }, 1000);
}

function complexOperationPromise() {
    return new Promise((resolve, reject) => {
        complexOperation(
            (result1) => resolve({ result1 }), // Resolve with the first result
            (result2) => reject(result2) // Reject with the second result
        );
    });
}

Error Handling#

Ensure you handle errors appropriately by rejecting the Promise with any error encountered:

function fetchData(callback) {
    setTimeout(() => {
        const error = null; // or some condition that generates an error
        const data = "Some data";
        callback(error, data); // The callback signature
    }, 1000);
}

function fetchDataPromise() {
    return new Promise((resolve, reject) => {
        fetchData((error, data) => {
            if (error) {
                reject(new Error(error)); // Create a new Error object if necessary
            } else {
                resolve(data);
            }
        });
    });
}

5. Using async/await#

With Promises in place, you can use async/await for cleaner and more synchronous-looking code:

async function main() {
    try {
        const data = await fetchDataPromise();
        console.log("Data received:", data);
    } catch (error) {
        console.error("Error occurred:", error);
    }
}

main();

By following these steps, you can convert callback-based APIs to use Promises, making your asynchronous code easier to manage and understand.