[译]Javascript中的错误信息处理(Error handling)

本文翻译youtube上的up主kudvenkat的javascript tutorial播放单

源地址在此:

https://www.youtube.com/watch?v=PMsVM7rjupU&list=PL6n9fhu94yhUA99nOsJkKXBqokT3MBK0b

在Javascript中使用try/catch/finally来处理runtime的错误.这些runtime错误被称为exceptions.各种各样的原因都可能导致exception.比如,使用没有申明的变量或者方法都可能导致exception.

可能导致exceptions的Javascript语句都应该被包裹在try语句内.当try语句内的某一句导致exception的时候,控制权就立刻转移到catch语句内,然后跳过try语句内的剩下所有代码内容.

Javascript try catch例子:

try 
{
    // Referencing a function that does not exist cause an exception
    // 使用不存在的函数导致产生一个exception
    document.write(sayHello());
    // Since the above line causes an exception, the following line will not be executed
    // 因为以上的代码导致出现exception,所以这以下的代码将不会运行
　　 document.write("This line will not be executed");
}
// When an exception occurs, the control is transferred to the catch block
//当exception产生后,控制权就转交到了catch语句块中

catch (e) 
{
    document.write("Description = " + e.description + "[br/]");
    document.write("Message = " + e.message + "[br/]");
    document.write("Stack = " + e.stack + "[br/][br/]");
}
document.write("This line will be executed");

Output : // JavaScript try catch example.png

请注意:一个try语句块应该始终紧跟一个catch语句块或者一个finally语句块,或者两者兼有

finally语句块无论exception是否产生都会运行.其主要用处在于清理和释放脚本运行期间所占用的资源.举个例子,如果你的try语句块打开了一个文件且运行,而好死不死又发生了一个exception,那么finally语句块就是用来关闭文件的.

Javascript try catch finally例子:

try 
{
    // Referencing a function that does not exist cause an exception
    document.write(sayHello());
    // Since the above line causes an exception, the following line will not be executed
    document.write("This line will not be executed");
}
// When an exception occurs, the control is transferred to the catch block
catch (e) 
{
    document.write("Description = " + e.description + "[br/]");
    document.write("Message = " + e.message + "[br/]");
    document.write("Stack = " + e.stack + "[br/][br/]");
}
finally 
{
    document.write("This line is guaranteed to execute");
}

Output : JavaScript try catch finally example.png

Javascript中的格式错误与exceptions需要注意的是:

try/catch/finally语句块可以捕捉到exceptions,但是格式错误则无法被其捕捉到.

例子:以下代码中有一个格式错误-丢失了闭小括号.catch语句块则无法捕捉到这个错误

try 
{
    alert("Hello";
}
catch (e) 
{
    document.write("JavaScript syntax errors cannot be caught in the catch block");       
}

Javascript throw语句:用throw语句来捕捉自定义exceptions

Javascript throw exception例子:

JavaScript throw exception example :
function divide() 
{
    var numerator = Number(prompt("Enter numerator"));
    var denominator = Number(prompt("Enter denominator"));

    try 
    {
        if (denominator == 0) 
        {
            throw {
                error: "Divide by zero error",
                message: "Denominator cannot be zero"
            };
        }
        else 
        {
            alert("Result = " + (numerator / denominator));
        }

    }
    catch (e) 
    {
        document.write(e.error + "[br/]");
        document.write(e.message + "[br/]");
    }
}

divide();