按顺序执行多个异步函数的方法

function a(cb){
console.log('this is a1 ')
cb()
console.log('this is a2')
}
function b(cb){
console.log('this is b1')
cb('b')
console.log('this is b2')
}
function c(cb){
console.log('this is c1')
cb()
console.log('this is c2')
}
function d(cb){
console.log('this is d1')
cb()
console.log('this is d2')
}
function e(cb){
console.log('this is e1')
cb()
console.log('this is e2')
}
function f(){
console.log('this is f')
}
a(b.bind(null,c.bind(null,d.bind(null,e.bind(null,f)))))

this is a1 this is b1this is c1this is d1this is e1this is fthis is e2this is d2this is c2this is b2this is a2改变console.log与cb的位置还可以得到不同的结果，具体可以多实验