Converting Callback functions to Promises

promisify.js

// when 'func' is a function that expects callback as the first parameter
function promisify(func){
	return (...args) => {
		return new Promise((resolve,reject) => {
        	func.call(undefined, (err,res)=>{
            	if(err){
                	return reject(err);
				}
				return resolve(res);
			},...args)
		});
	}
}

//when 'func' expects the last arguement as the callback function
function promisify(func){
	return (...args) => {
		return new Promise((resolve,reject) => {
        	func.call(undefined, ...args, (err,res)=>{
            	if(err){
                	return reject(err);
				}
				return resolve(res);
			})
		});
	}
}



let setTimeoutPromise = promisify(setTimeout);
setTimeoutPromise(5000).then(()=> console.log("5 seconds elapsed"));