JavaScript中Promise处理异步的并行与串行

// 执行FN1返回一个promise实例，实例中管理了一个异步编程的代码，当定时器到时间后，才会把实例的状态改为成功
const fn1 = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(1);
        }, 1000);
    });
};
const fn2 = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(2);
        }, 2000);
    });
};
const fn3 = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(3);
        }, 3000);
    });
}; 


// 异步的“并行”：同时处理，相互之间没啥依赖
fn1().then(result => {
    console.log(result);
});
fn2().then(result => {
    console.log(result);
});
fn3().then(result => {
    console.log(result);
}); 

let p = Promise.all([Promise.resolve(100), fn1(), 200, fn3(), fn2()]); //浏览器默认会把200 变为一个成功的promsie实例
p.then(results => {
    console.log(`成功：${results}`); // 成功：100,1,200,3,2 顺序和最开始是一致的，不会考虑谁先成功
}).catch(reason => {
    console.log(`失败：${reason}`);
}); 

//new Error('xxx')不是promise实例=》Promise.resolve(new Error('xxx')) 值是Error对象
let p = Promise.all([Promise.resolve(100), fn1(), new Error('xxx'), Promise.reject(200), fn3(), fn2()]);
p.then(results => {
    console.log(`成功：${results}`);
}).catch(reason => {
    console.log(`失败：${reason}`); //失败：200 遇到错误直接返回
}); 

//=>等三个异步都成功做什么事情 所用时间3s
Promise.all([fn1(), fn2(), fn3()]).then(results => {
    console.log(`三个异步都成功了，分别的结果：${results}`);
}); 

fn1().then(result => {
    console.log(`第一个成功:${result}`);
    return fn2();
}).then(result => {
    console.log(`第二个成功:${result}`);
    return fn3();
}).then(result => {
    console.log(`第三个成功:${result}`);
}).catch(reason => {
    console.log(`只要其中一个失败，直接顺延到这里，其余剩下的请求就不发送了!`);
});

(async function () {
    let result = await fn1();
    console.log(`第一个成功:${result}`);

    result = await fn2();
    console.log(`第二个成功:${result}`);

    result = await fn3();
    console.log(`第三个成功:${result}`);
})();

async function fn() {
    return 10;
}
fn().then(result => {
    console.log(result);//输出10
}); 

async function fn() {
    return async function () {
        return 10;
    };
}
fn().then(result => {
    // result:async function () {...}  函数
    return result();
}).then(result => {
    console.log(result); //10
}); 

const fn1 = () => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(1);
        }, 1000);
    });
};
(async function () {
    let result = await fn1();
    console.log(result); 
  //下面代码可以执行，说明await后面的promise实例，它的状态已经是成功了，await的返回值就是当前promise实例的值
    console.log('OK');
})(); 


//=======================如果await后面的promise实例状态是失败的
const fn1 = () => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            reject(0);
        }, 1000);
    });
};
(async function () {
    let result = await fn1(); //如果await后面的promise实例状态是失败的，则下面代码永远都不会执行了
    console.log(result);
    console.log('OK');//如果是失败的则这句话不会执行
})();

//===================================================
(async function () {
    let a = await 10; //->Promise.resolve(10)
    console.log(a); //10
    let b = await Promise.resolve(20);
    console.log(b); //20
    try {
        let c = await Promise.reject(30); //Uncaught (in promise) 30
        console.log(c);
    } catch (err) {
        console.log(err); //30
    }
    console.log('OK'); //'OK'
})(); 

(async function () {
    let b = await Promise.resolve(20);
    console.log(b);
})()

console.log(1);
setTimeout(() => { console.log(2); });//宏1
console.log(3);
let p1 = new Promise(resolve => {//new Promise 立即会把executor函数执行 是同步
    console.log(4);
    resolve('A'); //执行resolve P1的状态是成功=》已知
    console.log(5);
});
console.log(6);
p1.then(result => {//.then是异步 已知状态 放进WebAPI中监听 但是因为是已知状态 方法肯定执行 则从WebAPI监听队列挪到EventQueue中等待
    console.log(result); //微1
});
console.log(7);
let p2 = new Promise(resolve => { //立即执行executor函数 setTimeout是异步则会放进WebAPI监听 宏2
    setTimeout(() => {
        resolve('B');
        console.log(10); //执行宏2=>@1 更改了p2的状态 @2之前存储的.then方法执行
    });
});
console.log(8);
p2.then(result => {//p2的状态是未知则会先存起来
    console.log(result);
});
console.log(9);

let body = document.body;
body.addEventListener('click', function () {
    Promise.resolve().then(() => {
        console.log(1);
    });
    console.log(2);
});
body.addEventListener('click', function () {
    Promise.resolve().then(() => {
        console.log(3);
    });
    console.log(4);
}); 

 async function async1() {
    console.log('async1 start');
    await async2();
    console.log('async1 end');
}
async function async2() {
    console.log('async2');
}
console.log('script start');
setTimeout(function () {
    console.log('setTimeout');
}, 0)
async1();
new Promise(function (resolve) {
    console.log('promise1');
    resolve();
}).then(function () {
    console.log('promise2');
});
console.log('script end');