Error first callback:

• It is like a design pattern.
• the first argument of the callback is always the error, and the second argument of the callback is always the successful response data.

fs.read("/foo.txt",(error, data)=>{
	if(!error) return   //write your error handling code
	console.log(data)
})

• If you declare a function with the async keyword, it allows the consumption of promises using the await keyword.
• You can't use the await keyword outside the async function.
• async gives an indication that it can be asynchronous. If you call any async function, it will execute normally(synchronous), but the moment it goes to asynchronous code then it starts behaving like typical promises(move back to caller).

example 1 :

async function test(){
	console.log("async func..") //2
	return "233"
}

console.log("started..") //1
test()
console.log("end..") //3

//output :
started..
async func..
end..

Example 2:

async function test() {
	console.log("async func.."); //2
	setTimeout(() => {
		console.log("timer completed.. "); //2
		return 878;
	}, 2000);
}

console.log("started.."); //1
test();
console.log("end.."); //3

//output:
started..
async func..
end..
timer completed..

snippet 1 :

function fetchData(url) {
	return new Promise((resolve, reject) => {
		console.log("starting downloading from url..");
		setTimeout(() => {
			console.log("download completed.. ");
			let response = "Dummy data";
			resolve(response);
		}, 4000);
		});
}

function writeFile(data) {
	return new Promise((resolve, reject) => {
		console.log("starting writing data..", data, "in a file");
		setTimeout(() => {
			let filename = "fileName.txt";
			console.log("file written successfully..");
			resolve(filename);
		}, 10000);
	});
}

function uploadFile(filename, newUrl) {
	return new Promise((resolve, reject) => {
		setTimeout(() => {
			console.log(`file ${filename} uploaded succesful on ${newUrl}`);
			let uploadResponse = "success";
			console.log("upload done..");
			resolve(uploadResponse);
		}, 10000);
	});
}

async function processing() {
	let downloadedData = await fetchData("www.google.com");
	console.log("1");
	let writtenData = await writeFile(downloadedData);
	console.log("2");
	let uploadFile = await writeFile(writtenData, "www.drive.com");
	console.log("3");
}

console.log("started");
processing();
console.log("end");

//output :
started
starting downloading from url..
end
download completed..
1
starting writting data.. Dummy data in a file
file written successfully..
2
starting writting data.. fileName.txt in a file
file written successfully..
3

snippet _ 2 :