首页 \ 问答 \ 什么'if(err)'在Javascript中精确测试?(What does 'if (err)' tests precisely in Javascript?)

什么'if(err)'在Javascript中精确测试?(What does 'if (err)' tests precisely in Javascript?)

许多回调函数通过检查err参数开始,如下所示:

function myCallback(err, result) {

    if ( err ) {

        // Handle error
        ...

    }

    ...

}

if (err)测试验证了什么? 是否检查对象是否为null ? 是否检查是否undefined ? 不应该为instanceof Error测试吗? 该测试何时返回true


Many callback functions start by checking an err parameter as following:

function myCallback(err, result) {

    if ( err ) {

        // Handle error
        ...

    }

    ...

}

What does the if (err) test verifies? Does it check whether the object is null? Does it check whether it is undefined? Shouldn't one test for instanceof Error? When does this test return true?


原文:https://stackoverflow.com/questions/24754567
更新时间:2020-03-23 22:25

最满意答案

在Node.js世界中,回调函数具有以下签名

function(error, result){}

如果没有错误,则error参数通常设置为null ,否则为相应的错误对象。

因此,检查if (err)检查我们是否确实有错误情况。 如果err对象为null ,则检查将失败(因为null是假的)否则执行错误处理代码。

不应该为instanceof Error测试吗?

它不一定是Error对象的实例。 我们甚至可以将对象文字作为错误对象传递,甚至可以传递表示错误数组的数组。

是否检查对象是否为空? 是否检查是否未定义?

如果错误对象为nullundefined ,则测试将评估为falsy。


In Node.js world, the callback functions have the following signature

function(error, result){}

If there is no error, then the error parameter is conventionally set to null, otherwise the corresponding error object.

So, the check if (err) checks if we really have an error situation or not. If the err object is null, then the check will fail (since null is falsy) otherwise the error handling code is executed.

Shouldn't one test for instanceof Error?

It is not necessarily an instance of Error object. We can even pass an object literal as an error object or even an array representing array of errors.

Does it check whether the object is null? Does it check whether it is undefined?

The test will evaluate to be falsy is the error object is either null or undefined.

2014-07-15

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