这篇文章主要讲一下如何串行执行一组异步任务，例如有下面几个任务，在这里我们用setTimeout模拟一个异步任务：
let taskA = () => setTimeout(() => console.log('run task A'), 100);
let taskB = () => setTimeout(() => console.log('run task B'), 50);
let taskC = () => setTimeout(() => console.log('run task C'), 150);

let taskA = () => setTimeout(() => console.log('run task A'), 100);
let taskB = () => setTimeout(() => console.log('run task B'), 50);
let taskC = () => setTimeout(() => console.log('run task C'), 150);
直接运行
taskA(); taskB(); taskC();
taskA(); taskB(); taskC();是达不到顺序执行A，B，C 的三个任务的效果的。首先我们看一下最传统的做法，通过回调的方式在一个任务执行完成之后调用下一个任务：
let taskA = setTimeout(() => {
console.log('run task A');
taskB();
}, 100);
let taskB = setTimeout(() => {
console.log('run task B');
taskC();
}, 50);
let taskC = setTimeout(() => {
console.log('run task B');
}, 150);

let taskA = setTimeout(() => {
console.log('run task A');
taskB();
}, 100);
let taskB = setTimeout(() => {
console.log('run task B');
taskC();
}, 50);
let taskC = setTimeout(() => {
console.log('run task B');
}, 150);
第二种方法是将每一个任务封装成一个返回Promise的函数, 然后使用使用Promise的链式调用达到串行执行的目的：
let taskA = () => new Promise((resolve, reject) => {
setTimeout(() => {

console.log('run task A');

resolve();
}, 100);
})
let taskB = () => new Promise((resolve, reject) => {
setTimeout(() => {

console.log('run task B');

resolve();
}, 50);
})
let taskC = () => new Promise((resolve, reject) => {
setTimeout(() => {

console.log('run task C');

resolve();
}, 150);
})
function runTasks2() {

console.log('tasks 2');

taskA().then(taskB).then(taskC);
}

let taskA = () => new Promise((resolve, reject) => {
setTimeout(() => {

console.log('run task A');

resolve();
}, 100);
})
let taskB = () => new Promise((resolve, reject) => {
setTimeout(() => {

console.log('run task B');

resolve();
}, 50);
})
let taskC = () => new Promise((resolve, reject) => {
setTimeout(() => {

console.log('run task C');

resolve();
}, 150);
})
function runTasks2() {

console.log('tasks 2');

taskA().then(taskB).then(taskC);
}
假设任务的数量不确定，可以通过下面的方式来执行：
function runTasks3(tasks) {

console.log('tasks 3');

let pro = tasks[0]();

for (let i = 1; i < tasks.length; i++) {

pro.then(tasks[i]);

}
}

function runTasks3(tasks) {

console.log('tasks 3');

let pro = tasks[0]();

for (let i = 1; i < tasks.length; i++) {

pro.then(tasks[i]);

}
}
借助于es7的async和await,我们还可以对上面的函数一种写法：
async function runTasks3_1(tasks) {

for (let i = 0; i < tasks.length; i++) {

await tasks[i]();

}
}

async function runTasks3_1(tasks) {

for (let i = 0; i < tasks.length; i++) {

await tasks[i]();

}
}
在文章的最后我们自己来实现一个串行执行器， 用于执行一组串行任务：
function async(tasks) {

const count = tasks.length;

let index = 0;

const next = () => {

if (index >= count) return;

const task = tasks[index++];

task(next);

}

next(0);
}

function async(tasks) {

const count = tasks.length;

let index = 0;

const next = () => {

if (index >= count) return;

const task = tasks[index++];

task(next);

}

next(0);
}
函数的使用方式如下：

async([

next => setTimeout(() => { console.log('taskA ...'); next() }, 100),

next => setTimeout(() => { console.log('taskB ...'); next() }, 50),

next => setTimeout(() => { console.log('taskC ...'); next() }, 30)
]);


async([

next => setTimeout(() => { console.log('taskA ...'); next() }, 100),

next => setTimeout(() => { console.log('taskB ...'); next() }, 50),

next => setTimeout(() => { console.log('taskC ...'); next() }, 30)
]);
在每一个子任务中我们通过调用next函数继续执行下一个子任务。在具体的使用中可能会遇到函数之间传递参数的情况，即前一个任务的执行结果需要作为下一个任务的入参，这些都可以对上面的例子稍作修改就可以了~~