带有拒绝调用与抛出错误的 Promise 构造函数

Question

在以下代码中：

var p1 = new Promise(function (resolve, reject) {
    throw 'test1';
});

var p2 = new Promise(function (resolve, reject) {
    reject('test2');
});

p1.catch(function (err) {
    console.log(err); // test1
});

p2.catch(function (err) {
    console.log(err); // test2
});

使用Promise api 中的reject（在p2）和使用throw 引发错误（在p1）之间有什么区别吗？

一模一样？

如果相同，为什么我们需要reject 回调呢？

Answer 1

一个非常有趣的观察是，如果您使用throw，它将首先由reject 处理程序处理，如果没有拒绝处理程序，则由error 处理程序处理。

带有拒绝处理程序块

var allowed = false;
var p1 = new Promise(
function(resolve, reject) {
  if (allowed)
    resolve('Success');
  else
//         reject('Not allowed');
    throw new Error('I threw an error')
})

p1.then(function(fulfilled) {
console.log('Inside resolve handler, resolved value: ' + fulfilled);
}, function(rejected) {
console.log('Inside reject handler, rejected value: ' + rejected);
}).catch(function(error) {
console.log('Inside error handler, error value: ' + error);
})

没有拒绝处理程序块

var allowed = false;
var p1 = new Promise(
function(resolve, reject) {
  if (allowed)
    resolve('Success');
  else
//         reject('Not allowed');
    throw new Error('I threw an error')
})

p1.then(function(fulfilled) {
console.log('Inside resolve handler, resolved value: ' + fulfilled);
}).catch(function(error) {
console.log('Inside error handler, error value: ' + error);
})

此外，catch 块将能够捕获resolve 处理程序中抛出的任何错误。

var allowed = true;
var p1 = new Promise(
function(resolve, reject) {
  if (allowed)
    resolve('Success');
  else
//         reject('Not allowed');
    throw new Error('I threw an error')
})

p1.then(function(fulfilled) {
console.log('Inside resolve handler, resolved value: ' + fulfilled);
throw new Error('Error created inside resolve handler block');
}).catch(function(error) {
console.log('Inside error handler, error value: ' + error);
})

看起来最好使用throw，除非您在运行一些异步任务时不能这样做，否则您必须将reject 回调传递给异步函数。但是有一个解决方法，那就是承诺你的异步功能。更多关于https://stackoverflow.com/a/33446005

Answer 2

我知道这有点晚了，但我真的认为这些答案中的任何一个都不能完全回答我发现这个问题时的问题，这是一个更完整的例子。

var p1 = new Promise(function (resolve, reject) {
    throw 'test 1.1'; //This actually happens
    console.log('test 1.1.1'); //This never happens
    reject('test 1.2'); //This never happens because throwing an error already rejected the promise
    console.log('test 1.3'); //This never happens
});

var p2 = new Promise(function (resolve, reject) {
    reject('test 2.1'); //This actually happens
    console.log('test 2.1.1'); //This happens BEFORE the Promise is rejected because reject() is a callback
    throw 'test 2.2'; //This error is caught and ignored by the Promise
    console.log('test 2.3'); //This never happens
});

var p3 = new Promise(function (resolve, reject) {
    setTimeout(function() { reject('test 3.1');}, 1000); //This never happens because throwing an error already rejected the promise
    throw('test 3.2'); //This actually happens
    console.log('test 3.3'); //This never happens
});

var p4 = new Promise(function (resolve, reject) {
    throw('test 4.1'); //This actually happens
    setTimeout(function() { reject('test 4.2');}, 1000); //This never happens because throwing an error already rejected the promise
    console.log('test 4.3'); //This never happens
});

var p5 = new Promise(function (resolve, reject) {
    setTimeout(function() { throw('test 5.1');}, 1000); //This throws an Uncaught Error Exception
    reject('test 5.2'); //This actually happens
    console.log('test 5.3'); //This happens BEFORE the Promise is rejected because reject() is a callback
});

var p6 = new Promise(function (resolve, reject) {
    reject('test 6.1'); //This actually happens
    setTimeout(function() { throw('test 6.2');}, 1000); //This throws an Uncaught Error Exception
    console.log('test 6.3'); //This happens BEFORE the Promise is rejected because reject() is a callback
});


p1.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test1
});

p2.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test2
});

p3.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test3
});

p4.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test4
});

p5.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test5
});

p6.then(function (resolve) {
    console.log(resolve, "resolved")
}, function (reject) {
    console.log(reject, "rejected")
}).catch(function (err) {
    console.log(err, "caught"); // test6
});

Answer 3

使用Promise api 中的reject（在p2）和使用throw 引发错误（在p1）之间有什么区别吗？

是的，你 cannot 异步使用 throw，而 reject 是一个回调。例如，一些超时：

new Promise(_, reject) {
    setTimeout(reject, 1000);
});

一模一样？

不，至少在您的语句后面有其他代码时不会。 throw 立即完成解析器功能，同时调用 reject 继续正常执行 - 在将承诺“标记”为被拒绝之后。

此外，如果您throw 错误对象，引擎可能会提供不同的异常调试信息。

对于您的具体示例，您是对的，p1 和 p2 与外部无法区分。

Answer 4

不，没有，两者完全相同。唯一的区别以及我们需要 reject 的原因是当您需要异步拒绝时 - 例如，如果您正在转换基于回调的 API，它可能需要发出异步错误信号。

var p = new Promise(function(resolve, reject){
    someCallbackApi(function(err, data){
        if(err) reject(err); // CAN'T THROW HERE, non promise context, async.
        else resolve(data);
    });
});