How to Convert Callback to Promise in JavaScript ?

Asynchronous programming in JavaScript often involves the use of callbacks. However, callbacks can lead to callback hell and make the code harder to read and maintain. Promises provide a cleaner way to handle asynchronous operations. Converting existing callback-based code to use promises can improve readability and maintainability.

Using the Promise constructor

This approach involves manually creating a promise around the asynchronous operation. It requires creating a new promise and resolving or rejecting it based on the outcome of the asynchronous operation.

Syntax:

function promisifiedFunction(arg1, arg2) {
    return new Promise((resolve, reject) => {
        myFunctionWithCallback(arg1, arg2, (error, result) => {
            if (error) {
                reject(error);
            } else {
                resolve(result);
            }
        });
    });}

Example: In this, the promise-based function is used to fetch data from the API.

JavaScript

function fetchDataWithCallback(url, callback) {
    setTimeout(() => {
        const data = {
            message: 'Data fetched successfully',
            url: url
        };
        callback(null, data);
    }, 1000);


    function fetchDataPromise(url) {
        return new Promise((resolve, reject) => {
            fetchDataWithCallback(url, (error, data) => {
                if (error) {
                    reject(error);
                } else {
                    resolve(data);
                }
            });
        });
    }

    fetchDataPromise('https://api.example.com/data')
        .then(data => console.log('Data:', data))
        .catch(error => console.error('Error:', error));

Output:

Output

Using a custom promise-based wrapper

This approach involves creating a custom wrapper function around the existing callback-based function.
Inside the wrapper function, a new promise is created, and the original function is called with a callback.
The promise is resolved or rejected based on the callback’s result.

Syntax:

function promisifiedFunction(arg1, arg2) {
    return new Promise((resolve, reject) => {
        originalFunctionWithCallback(arg1, arg2, (error, result) => {
            if (error) {
                reject(error);
            } else {
                resolve(result);
            }
        });
    });}

Example: In this, the promise-based function is used to fetch data from the API.

JavaScript

function fetchDataWithCallback(url, callback) {
    setTimeout(() => {
        const data = {
            message: 'Data fetched successfully',
            url: url
        };
        callback(null, data);
    }, 1000);
}

function fetchDataPromise(url) {
    return new Promise((resolve, reject) => {
        fetchDataWithCallback(url, (error, data) => {
            if (error) {
                reject(error);
            } else {
                resolve(data);
            }
        });
    });
}

fetchDataPromise('https://api.example.com/data')
    .then(data => console.log('Data:', data))
    .catch(error => console.error('Error:', error));

Output:

Output