承诺：那么vs那么+赶上 [英] Promise : then vs then + catch

问题描述

以下两个代码之间是否有任何区别？

Is there any difference between the 2 followings codes ?

myPromise.then(function() {
    console.log('success');
}).catch(function() {
    console.log('error');
});

myPromise.then(function() {
    console.log('success');
}, function() {
    console.log('error');
});

我知道然后和 catch 使用回调中的值返回返回已解决或拒绝的新promise。但是我看到了网络上的2个代码，我很好奇两个代码之间的真正区别。

I know then and catch returns new promises resolved or rejected with the value returns in the callback. But i see the 2 codes around the web and i'm curious about the real differences between the 2 codes.

推荐答案

在你的当前代码，它们的行为相同，因为 console.log（'success'）; 不会失败。

In your current code, they act the same because console.log('success'); will not fail.

但是，如果你写这样的东西......

However, if you write something like this...

myPromise.then(function() {
   // Some error may happen
   throw('An exception that would be caught');
}).catch(function() {
    console.log('error');
});
// Is the same as this, the errHandle tries to catch any unhandled error
// from previous result.
myPromise.then(func, null).then(null, errHandle);


myPromise.then(function() {
   // Some error may happen
   throw('An unhandled exception.');
}, function() {
    // This won't log the error if it happens in the 
    // some error may happen block.
    console.log('error');
});
// Is the same as this, the errHandle will handle errors from previous result,
// but it won't handle errs in func.
myPromise.then(func, errHandle)

第二种形式无法捕捉到该错误，第一个可以。

The second form can't catch that error, while the first can.

测试片段：

Snippet for test:

// An function that may error and throw exception.
function funcThatThrows(test) {
  throw(`oops - error in test ${test}`);
}
function errHandler(exception) {
  console.log('We got an exception: ', exception);
}
// Expect: We got an exception:  oops - error in test 1
Promise.resolve(1).then(funcThatThrows).catch(errHandler);
// Is the same as below, the errHandler tries to catch any unhandled error
// from previous result.
// Expect: We got an exception:  oops - error in test 2
Promise.resolve(2).then(funcThatThrows, null).then(null, errHandler);

// If put the function and handler in the same then, the exception won't be caught.
// Expect: Uncaught (in promise) oops - error in test 3
Promise.resolve(3).then(funcThatThrows, errHandler);

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