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reject api中的p2（Promise）与使用p1使用throw抛出错误（reject}之间有什么区别吗？

它完全一样吗？

如果相同，为什么我们需要{{1}}回调？

Answer 1

使用reject api中的p2（Promise）与使用p1使用throw抛出错误（throw}之间有什么区别吗？

是的，您cannot异步使用reject，而new Promise(_, reject) { setTimeout(reject, 1000); });是回调。例如，一些超时：

throw

它完全一样吗？

不，至少在其他代码跟随您的陈述时不会。 reject立即完成解析器功能，同时调用throw继续正常执行 - 在将“承诺”标记为“已拒绝”之后。

此外，如果p1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，您认为p2和{{1}}与外界无法区分。

Answer 2

不，没有，两者完全相同。我们需要reject的唯一区别和原因是当您需要异步拒绝时 - 例如，如果您要转换基于回调的API，则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);
    });
});

Answer 3

我知道这有点晚了，但我并不认为这些答案中的任何一个完全回答了我在发现这个问题时所遇到的问题，这是一个更全面的例子。

＆＃13;

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
});

＆＃13;

Answer 4

一个非常有趣的观察是，如果你使用throw，它将首先由reject处理程序＆amp;处理。如果没有拒绝处理程序，那么error处理程序。

使用拒绝处理程序阻止

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，除非你不能运行某些异步任务，否则你必须将reject回调传递给async函数。但是有一个解决方法，那就是宣传你的异步功能。更多关于https://stackoverflow.com/a/33446005

承诺构造函数与拒绝调用vs抛出错误

4 个答案: