1.如何修改对象的状态？
resolve函数 resolve(‘ok’);
reject函数 reject(‘error’);
throw ‘出问题了’
2.Promise可以指定多个回调
3.改变promise状态和指定回调函数谁先谁后？
都有可能，正常情况下先指定回调再改变状态（异步），但也可以先改状态再指定回调（同步状态）。
let p = new Promise((resolve,reject) => {
resolve("OK");
});
let result = p.then(value => {
//result的结果为失败
return new Promise((resolve,reject) => {
rejected("error");
});
4.串联多个任务
let p = new Promise((resolve,reject) => {
setTimeout(() => {
resolve("OK");
},1000);
});
p.then(value => {
return new Promise((resolve,reject) => {
resolve("success");
});
}).then(value => {
console.log(value); //success
}).then(value => {
console.log(value); //undefined
})
此处第二次console.log(value)输出结果为undefined, 因为then的返回结果是一个Promise，Promise的状态由它指定的函数状态的返回值来决定，这里指定的是前一个then，前一个then返回值没声明，所以输出undefined
5.Promise异常穿透现象
只需要再最后指定一个失败的回调，中间有任务失败，最后也会报出异常
let p = new Promise((resolve,reject) => {
setTimeout(() => {
resolve("OK");
},1000);
});
p.then(value => {
return new Promise((resolve,reject) => {
resolve("success");
});
}).then(value => {
console.log('111'); //success
}).then(value => {
console.log('222'); //undefined
}).then(value => {
console.log('333'); //undefined
}).catch(reason => {
console.warn(reason);
})
6.如何终端Promsie链
返回一个状态为padding的promise
let p = new Promise((resolve,reject) => {
setTimeout(() => {
resolve("OK");
},1000);
});
p.then(value => {
return new Promise((resolve,reject) => {
resolve("success");
});
}).then(value => {
console.log('111');
return new Promise(() => {
});
}).then(value => {
console.log('222');
}).then(value => {
console.log('333');
}).catch(reason => {
console.warn(reason);
})