Promise 多個異步順序執行

const fs = require('fs');

// fs.readFile('./1.txt', 'utf8', (err, result1) => {
// 	console.log(result1)
// 	fs.readFile('./2.txt', 'utf8', (err, result2) => {
// 		console.log(result2)
// 		fs.readFile('./3.txt', 'utf8', (err, result3) => {
// 			console.log(result3)
// 		})
// 	})
// });

function p1 () {
	return new Promise ((resolve, reject) => {
		fs.readFile('./1.txt', 'utf8', (err, result) => {
			resolve(result)
		})
	});
}

function p2 () {
	return new Promise ((resolve, reject) => {
		fs.readFile('./2.txt', 'utf8', (err, result) => {
			resolve(result)
		})
	});
}

function p3 () {
	return new Promise ((resolve, reject) => {
		fs.readFile('./3.txt', 'utf8', (err, result) => {
			resolve(result)
		})
	});
}

p1().then((r1)=> {
	console.log(r1);
	return p2();
})
.then((r2)=> {
	console.log(r2);
	return p3();
})
.then((r3) => {
	console.log(r3)
})