Promisify func

promisify.js

function promisify(func) {

    // A new function which returns promise
    return function (...args) {

        return new Promise((resolve, reject) => {
            // Create our custom callback function which will original function will call
            let callback = function (err, response) {
                // On recieving callback it will resolve or reject the promise
                if (err) return reject(err);
                return resolve(response);
            }
            args.push(callback);
            func.call(this, ...args);
        })

    }
}