Javascript承诺:我可以知道Promise链的哪一部分引起错误吗? [英] Javascript Promises : Can I know which part of promise chain caused error?
问题描述
(请原谅我的英语)
我现在正在学习有关JavaScript的Promise。
下面的示例代码是用于node.js(我的node.js版本是v10.0.0 )的简单javascript代码,该代码使用promise链异步读取和解析JSON文件。
I am learning about javascript promises, now. Below sample code is a simple javascript code for node.js(my node.js version is v10.0.0), which asynchronously reads and parses a JSON file using promise chain.
const fs = require("fs");
function readFileAsync(filename) {
return new Promise((resolve, reject) => {
fs.readFile(filename, 'utf8', (error, result) => {
if (error)
reject(error);
else
resolve(result);
});
});
}
readFileAsync('test.json')
.then(res => JSON.parse(res))
.then(res => { console.log('JSON=', res); })
.catch(err => { console.log(err.message); });
我发现此示例代码生成不同格式的错误消息。
例如,如果找不到'test.json',则错误消息为...
I found that this sample code generates different formats of error messages. For example, if it cannot find 'test.json', the error message is...
ENOENT: no such file or directory, open '/home/node/test.json'
如果无法解析'test.json',错误消息是...
If it cannot parse 'test.json', the error message is...
Unexpected token / in JSON at position 31
我想修改示例代码以生成始终包含JSON文件名的错误消息格式。
为此,首先我应该知道promise链的哪一部分导致了错误。我怎么知道?
I want to modify the sample code to generate same format of error message always containing JSON file name.
To do so, firstly I should know which part of promise chain caused error. How can I know?
推荐答案
有两种方法可以到达所需的位置。
There are two ways to arrived what you want.
Promise。然后
有两个参数,请参见下面的代码和您可以在此处获取更多信息
Promise.then
has two arguments, see below code and you can get more information here
readFileAsync('test.json')
.then(res => JSON.parse(res))
.then(res => { console.log('JSON=', res); }, error => {
// here can catch error of previous then function
});
另一种方法是修改函数 readFileAsync
Another way is modify the function readFileAsync
function readFileAsync(filename) {
return new Promise(resolve => {
fs.readFile(filename, (error, result) => {
if (error)
resolve(null); // you can resolve whatever you want
else
resolve(result);
});
});
}
和 .catch ()
不会捕获 readFileAsync
的任何错误。
And .catch()
will not catch any error of readFileAsync
.
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