promise顺序执行的几种方法

const p1 = new Promise(resolve => {
   setTimeout(() => {
        resolve(1)
    }, 1000)
})

const p2 = new Promise(resolve => {
    setTimeout(() => {
        resolve(2)
    }, 3000)
})

const p3 = new Promise(resolve => {
    setTimeout(() => {
        resolve(3)
    }, 2000)
})

const p4 = new Promise(resolve => {
    setTimeout(() => {
        resolve(4)
    }, 1500)
})

const ps = [p1, p2, p3, p4]

// 方式一，递归

const run = (p) => {
    p.then((d) => {
        console.log(d)
        const nextP = ps[ps.findIndex(f => f === p) + 1]
        if (nextP) {
            run(nextP)
        }
    })
}
run(ps[0])

// 方式二，generator 函数，也是 co 库核心

const run = (gen) => {
    const it = gen()
    let ret = it.next()
    function step () {
        if (!ret.done) {
            ret.value.then(d => {
                console.log(d)
                ret = it.next(d)
                step()
            })
        }
    }
    step()
}
run(function *() {
    for (let p of ps) {
        yield p
    }
})

// 方式三，组成 then 链

const run = () => {
    let p = Promise.resolve()
    ps.forEach(item => {
        p = p.then(() => item).then(console.log)
    })
}
run()

// 方式四，两个函数互相调用，一个负责传递数据，一个负责执行

const run = (p, cb) => {
    if (p) {
        p.then(d => {
            console.log(d)
            cb()
        })
    }
}
const pick = () => {
    run(ps.shift(), pick)
}
pick()

// 方式五，async, await

const run = async function () {
    for (let p of ps) {
        const r = await p
        console.log(r)
    }
}
run()