当promise改变为对应状态时都会调用
const p = new Promise((resolve, reject) => {
// resolve(1) // promise变为resolved成功状态
// reject(2) // promise变为rejected失败状态
// throw new Error('出错了') // 抛出异常, promise变为rejected失败状态, reason为 抛出的error
throw 3 // 抛出异常, promse变为rejected失败状态, reason为 抛出的3
})
p.then(
value => {
},
reason => {
console.log('reason', reason) // reason3
}
)
p.then(
value => {
},
reason => {
console.log('reason2', reason) // reason23
}
)
常规: 先指定回调函数, 后改变的状态
new Promise((resolve, reject) => {
setTimeout(() => {
resolve(1) // 后改变的状态(同时指定数据), 异步执行回调函数
}, 1000);
}).then(// 先指定回调函数, 保存当前指定的回调函数
value => {},
reason => {console.log('reason', reason)}
)
如何先改状态, 后指定回调函数
new Promise((resolve, reject) => {
resolve(1) // 先改变的状态(同时指定数据)
}).then(// then为同步 后指定回调函数, 异步执行回调函数
value => {console.log('value2', value)},
reason => {console.log('reason2', reason)}
)
console.log('-------')
const p = new Promise((resolve, reject) => {
setTimeout(() => {
resolve(1) // 后改变的状态(同时指定数据), 异步执行回调函数
}, 1000);
})
setTimeout(() => {
p.then(
value => {console.log('value3', value)},
reason => {console.log('reason3', reason)}
)
}, 1100);
简单表达: 由then()指定的回调函数执行的结果决定
详细表达: 1、如果抛出异常, 新promise变为rejected, reason为抛出的异常 2、如果返回的是非promise的任意值, 新promise变为resolved, value为返回的值 3、如果返回的是另一个新promise, 此promise的结果就会成为新promise的结果
new Promise((resolve, reject) => {
// resolve(1)
reject(1)
}).then(
value => {
console.log('onResolved1()', value)
},
reason => {
console.log('onRejected1()', reason)
}
).then(
value => {
console.log('onResolved2()', value)
},
reason => {
console.log('onRejected2()', reason)
}
)
// 输出 onRejected1() 1 与 onResolved2() undefined
new Promise((resolve, reject) => {
resolve(1)
// reject(1)
}).then(
value => {
console.log('onResolved1()', value)
return 2
},
reason => {
console.log('onRejected1()', reason)
}
).then(
value => {
console.log('onResolved2()', value)
},
reason => {
console.log('onRejected2()', reason)
}
)
// 输出 onResolved1() 1 与 onResolved2() 2
new Promise((resolve, reject) => {
// resolve(1)
reject(1)
}).then(
value => {
console.log('onResolved1()', value)
},
reason => {
console.log('onRejected1()', reason)
throw 5
}
).then(
value => {
console.log('onResolved2()', value)
},
reason => {
console.log('onRejected2()', reason)
}
)
// 输出 onRejected1() 1 与 onRejected2() 5
new Promise((resolve, reject) => {
setTimeout(() => {
console.log('执行任务1(异步)')
resolve(1)
}, 1000)
}).then(
value => {
console.log('任务1的结果: ', value)
console.log('执行任务2(同步)')
return 2
}
).then(
value => {
console.log('任务2的结果:', value)
return new Promise((resolve, reject) => {
// 启动任务3(异步)
setTimeout(() => {
console.log('执行任务3(异步))')
resolve(3)
}, 1000)
})
}
).then(
value => {
console.log('任务3的结果: ', value)
}
)
new Promise((resolve, reject) => {
// resolve(1)
reject(1)
}).then(
value => {
console.log('onResolved1()', value)
return 2
},
// reason => {throw reason}
).then(
value => {
console.log('onResolved2()', value)
return 3
},
reason => {
throw reason
}
).then(
value => {
console.log('onResolved3()', value)
},
reason => Promise.reject(reason)
).catch(reason => {
console.log('onReejected1()', reason)
// throw reason
// return Promise.reject(reason)
return new Promise(() => {
}) // 返回一个pending的promise 中断promise链
}).then(
value => {
console.log('onResolved3()', value)
},
reason => {
console.log('onReejected2()', reason)
}
)