多个Promise顺序执行的解决思路

问题：多个延时器顺序执行，依次打印

function a() {
	setTimeout(() => {
		console.log(1)
	}, 200)
}

function b() {
	setTimeout(() => {
		console.log(2)
	}, 10)
}

function c() {
	setTimeout(() => {
		console.log(3)
	}, 30)
}

// a()
// b()
// c()
// console.log(4)

解决方案

function a() {
	return new Promise((resolve, reject) => {
		setTimeout(resolve, 200, 1)
	})
}

function b() {
	return new Promise((resolve, reject) => {
		setTimeout(resolve, 10, 2)
	})
}

function c() {
	return new Promise((resolve, reject) => {
		setTimeout(resolve, 30, 3)
	})
}

1. 使用回调函数解决

a().then(res => {
	console.log(res)
	b().then(res => {
		console.log(res)
		c().then(res => {
			console.log(res)
			console.log(4)
		})
	})
})

2. 使用promise/then链式调用

Promise.resolve().then(() => a())
.then(res => console.log(res))
.then(() => b())
.then(res => console.log(res))
.then(() => c())
.then(res => console.log(res))
.then(() => console.log(4))

3. 使用async/await解决

async function log(a, b, c) {
	const args = [].slice.call(arguments, 0)
	for (let item of args) {
		let res = await item()
		console.log(res)
	}
	console.log(4)
}
log(a, b, c)