在节点js

Question

我的应用程序中有两个文件， DesignFactory.js：

var fs = require('fs');
var dotenv = require('dotenv');
dotenv.load();
var designtokenfile = require ('./designtokenfile');
var designtokendb = require ('./designtokendb');
var TYPE=process.env.TYPE;
var DesignFactory={};
DesignFactory.storeDesign = function(TYPE) {
if (TYPE == 'file') {
  var data=design.designtokenfile.load();
  console.log(data);

} else if (TYPE == 'db') {
  return designtokendb;
}
};

module.exports.design=DesignFactory;

现在，我有另一个designtokenfile.js文件，  designtokenfile.js：

var fs = require('fs');
var load = function() {
fs.readFile('output.txt', 'utf8', function (err,data) {

  return data;
 if (err) {
   return console.log(err);
 }
    });

 };

 module.exports.load=load;

所以我的问题是无法从load方法返回数据。当我在从load函数返回的storeDesign方法中打印数据时，它显示undefined。 但我想在storeDesign方法中输出output.txt的内容。 请帮帮我。

Answer 1

而不是：

var load = function() {
  fs.readFile('output.txt', 'utf8', function (err, data) {
    return data;
     if (err) {
       return console.log(err);
     }
  });
};

由于永远无法联系到if之后的return，

无法正常工作，请使用此功能：

var load = function(cb) {
  fs.readFile('output.txt', 'utf8', function (err,data) {
     if (err) {
       console.log(err);
       return cb(err);
     }
     cb(null, data);
  });
};

并像这样使用它：

load((err, data) => {
  if (err) {
    // handle error
  } else {
    // handle success
  }
});

或者使用它：

var load = function(cb) {
  return new Promise(resolve, reject) {
     fs.readFile('output.txt', 'utf8', function (err, data) {
       if (err) {
         console.log(err);
         reject(err);
       }
       resolve(data);
     });
  });
};

并像这样使用它：

load().then(data => {
    // handle success
}).catch(err => {
    // handle error
});

换句话说，您无法将回调值返回到异步函数。您的函数需要进行回调或返回承诺。

你需要更多地了解Node的异步性，关于promises和回调。我可以推荐在Stack Overflow上有很多的问题和答案：

• promise call separate from promise-resolution
• Q Promise delay
• Return Promise result instead of Promise
• Exporting module from promise result
• What is wrong with promise resolving?
• Return value in function from a promise block
• How can i return status inside the promise?
• Should I refrain from handling Promise rejection asynchronously?
• Is the deferred/promise concept in JavaScript a new one or is it a traditional part of functional programming?
• How can I chain these functions together with promises?
• Promise.all in JavaScript: How to get resolve value for all promises?
• Why Promise.all is undefined
• function will return null from javascript post/get
• Use cancel() inside a then-chain created by promisifyAll
• Why is it possible to pass in a non-function parameter to Promise.then() without causing an error?
• Implement promises pattern
• Promises and performance
• Trouble scraping two URLs with promises
• http.request not returning data even after specifying return on the 'end' event
• async.each not iterating when using promises
• jQuery jqXHR - cancel chained calls, trigger error chain
• Correct way of handling promisses and server response
• Return a value from a function call before completing all operations within the function itself?
• Resolving a setTimeout inside API endpoint
• Async wait for a function
• JavaScript function that returns AJAX call data
• try/catch blocks with async/await
• jQuery Deferred not calling the resolve/done callbacks in order
• Returning data from ajax results in strange object
• javascript - Why is there a spec for sync and async modules?
• Return data after ajax call success

Answer 2

fs.readFile ist asynchrone，因此您需要传递回调函数或使用fs.readFileSync

Answer 3

由于异步性质，您获得undefined。请尝试以下代码：

var fs = require('fs');
var load = function(callback) {
fs.readFile('output.txt', 'utf8', function (err,data) {

  //return data;
   callback(null, data);
 if (err) {
   callback("error", null);
 }
    });

 };

 module.exports.load=load;

var fs = require('fs');
var dotenv = require('dotenv');
dotenv.load();
var designtokenfile = require ('./designtokenfile');
var designtokendb = require ('./designtokendb');
var TYPE=process.env.TYPE;
var DesignFactory={};
DesignFactory.storeDesign = function(TYPE) {
if (TYPE == 'file') {
  var data=design.designtokenfile.load(function(err, res){
   if(err){
   } else {
    console.log(data);
   }
}); 

} else if (TYPE == 'db') {
  return designtokendb;
}
};

module.exports.design=DesignFactory;