reject状态传递

reject状态传递途中会跳过成功处理函数

new Promise((resolve, reject) => {
        reject();
    })
    .then(() => { console.log('1.ok') }) //跳过
    .then(() => { console.log('2.ok') }, () => { console.log('2.fail') }) //如果不返回新的Promise，默认会回到成功状态
    .then(() => { console.log('3.ok') }, () => { console.log('3.fail') });

输出：

2.fail
3.ok

在then中返回的新Promise中reject也一样

new Promise((resolve, reject) => {
        resolve();
    })
    .then(() => { console.log('1.ok') })
    .then(() => {
        return new Promise((resolve, reject) => {
            reject();
        });
    })
    .then(() => { console.log('2.ok') }) //跳过
    .then(() => { console.log('3.ok') }, () => { console.log('3.fail') });

输出：

1.ok
3.fail

通过将成功处理函数置为null插入可选的流程

new Promise((resolve, reject) => {
        //登录后检查登录状态，若已登录则resolve，否则reject
        console.log('running');
        console.log('already logged in');
        resolve();
    })
    .then(null, msg => {
        //登录
        console.log('msg: ' + msg);
        console.log('login');
    })
    .then(() => {
        //继续
        console.log('go on');
    });

输出：

running
already logged in
go on

通过reject执行可选的流程

new Promise((resolve, reject) => {
        //登录后检查登录状态，若已登录则resolve，否则reject
        console.log('running');
        reject('not logged in');
    })
    .then(null, msg => {
        //登录
        console.log('msg: ' + msg);
        console.log('login');
    })
    .then(() => {
        //继续
        console.log('go on');
    });

输出：

running
msg: not logged in
login
go on

抛出和处理错误

抛出错误会触发失败处理函数，后面的catch捕获不到

new Promise((resolve, reject) => {
        throw new Error('failed');
    })
    .then(() => { console.log('ok') }, () => { console.log('fail') /*执行这里*/ })
    .catch(err => {
        console.log(err); //捕获不到
    });

输出：

fail

使用catch捕获

new Promise((resolve, reject) => {
        throw new Error('failed');
    })
    .then(() => { console.log('ok') }) //没有失败处理函数
    .catch(err => {
        console.log('error catched: ' + err.message); //执行这里
    });

输出：

error catched: failed

不能在回调函数里抛出错误，捕获不到

new Promise(resolve => {
        console.log('running');
        setTimeout(() => {
            console.log('will throw error');
            throw new Error('failed');
        }, 1000);
    })
    .catch(err => {
        console.log('error catched'); //捕获不到
    });

输出：

running
will throw error
/Users/Tom/Desktop/Projects/_tests/PromiseTest/main.js:40
            throw new Error('failed');
            ^

Error: failed
    at Timeout.setTimeout (/Users/Tom/Desktop/Projects/_tests/PromiseTest/main.js:40:19)
    at ontimeout (timers.js:380:14)
    at tryOnTimeout (timers.js:244:5)
    at Timer.listOnTimeout (timers.js:214:5)