promise

• Promise 对象用于表示一个异步操作的最终完成 (或失败)及其结果值。
  

• promise 有静态方法：race，all，reject，resolve, allSettled, any

• promise原型上的方法：then，catch，finally

• 一个 Promise 必然处于以下几种状态之一：

  • pending：初始状态，既没有被兑现，也没有被拒绝。
  • fulfilled：操作成功完成。
  • rejected：操作失败。

• 例子1

var a = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve({a: 1});
    }, 1000);
});

var b = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve({a: 1});
    }, 1200);
});

var aa = a.then((v) => {
    console.log(v, 'aa');
});

var bb = b.then((v) => {
    console.log(v, 'bb');
});

var aaa = aa.then((v) => {
    console.log(v, 'aaa');
});

var bbb = bb.then((v) => {
    console.log(v, 'bbb');
});

Promise.all([aa, bb]).then(data => {
    console.log('all', data);
});

Promise.all([a, b]).then(data => {
    console.log('a b all', data);
});

• 例子2

var a = new Promise((resolve, reject) => {
    setTimeout(() => {
        reject({a: 1});
    }, 1000);
});

var b = new Promise((resolve, reject) => {
    setTimeout(() => {
        reject({a: 1});
    }, 1200);
});

var aa = a.then((v) => {
    console.log(v, 'aa');
}).catch(() => {});

var bb = b.then((v) => {
    console.log(v, 'bb');
}).catch(() => {});

var aaa = aa.then((v) => {
    console.log(v, 'aaa');
})

var bbb = bb.then((v) => {
    console.log(v, 'bbb');
});

Promise.all([aa, bb]).then(data => {
    console.log('all', data);
});

Promise.all([a, b]).then(data => {
    console.log('a b all', data);
});

• 例子3

var a = new Promise((resolve, reject) => {
    setTimeout(() => {
        reject({a: 1});
    }, 1000);
});

var b = new Promise((resolve, reject) => {
    setTimeout(() => {
        reject({a: 1});
    }, 1200);
});

var aa = a.then((v) => {
    console.log(v, 'aa');
}).catch(() => {});

var bb = b.then((v) => {
    console.log(v, 'bb');
}).catch(() => {});

var aaa = aa.then((v) => {
    console.log(v, 'aaa');
})

var bbb = bb.then((v) => {
    console.log(v, 'bbb');
});

Promise.allSettled([aa, bb]).then(data => {
    console.log('all', data);
});

Promise.allSettled([a, b]).then(data => {
    console.log('a b all', data);
});