在节点js中返回数据

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加载方法返回数据。当我在 load 函数返回的 storeDesign 方法中打印数据时，它显示未定义。 但我想要 storeDesign 方法中的 output.txt 的内容。 请帮帮我。

Answer 1

代替：

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

因为永远无法到达return 之后的if，所以无法工作，请使用：

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
  }
});

或者使用这个：

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，您不能从回调函数中将值返回给异步函数。你的函数要么需要回调，要么返回一个 Promise。

您需要更多地了解 Node 的异步特性、承诺和回调。我可以推荐很多关于 Stack Overflow 的问题和答案：

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Answer 2

fs.readFileist asynchrone 所以你需要传递一个回调函数或者使用fs.readFileSync

Answer 3

由于异步性质，您收到undefined。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;