Node.js 无法访问另一个 javascript 文件中的变量

Question

This is my first javascript file which is called search.js which is used to query through twitter posts.

var Twit = require('twit');
var config = require('./config') 

var T = new Twit(config); 

var params = {

   q: '#stackOverflow',

   count: 1

} 


var response = null; 

T.get('search/tweets', params, searchedData); 


function searchedData(err, data, response) {
   response = data
   console.log(response) //prints the post
   return response;

} 

The twitter posts are stored in the 'response' variable returned in the last function. When I print the response variable, it properly prints the post. I need to access that variable in my index.js which runs the server.

Here is my index.js file:

const mySearch = require('./search.js');
const express = require('express');

const app = express();

app.get('/', function(req, res) {
    res.send('Hello World');
})

app.listen(3000, function(){
    console.log("Server started on port 3000...");
    console.log(mySearch.response)//prints 'undefined'
})

Can anyone please help me? I tried looking online but I still can't put the pieces together.

Thanks!

Answer 1

Create a promise for the response object and export it.

Your T.get('search/tweets', params, searchedData) is async. So, you can't really export a variable from the scope of searchedData function. You either should import your twit module directly in your main server file. Or, you can create a promise that you can export and then import in the main file. Below is how you can do it using promises:

var Twit = require('twit');
var config = require('./config') 

var T = new Twit(config); 

var params = {
   q: '#stackOverflow',
   count: 1
} 

var postPromise = new Promise((resolve, reject) => {
     T.get('search/tweets', params, (err, data) => {
         if (err) {
              reject(err);
         } else {
            resolve(data)
         }
     }); 
});

module.export.postPromise = postPromise;

And then you can import this promise in your index.js file and do something like:

const getPosts = require('./search.js').postPromise;
const express = require('express');

const app = express();

app.get('/posts', function(req, res) {
      getPosts.then(posts => res.status(200).json(posts));
})

...

Answer 2

module.exports.getTweets = function(callback) {
    var Twit = require('twit');
    var config = require('./config')

    var T = new Twit(config);

    var params = {

        q: '#stackOverflow',

        count: 1

    }

    T.get('search/tweets', params, callback);
}

You should use callback chaining to get the tweets properly.

const mySearch = require('./search.js');
const express = require('express');

const app = express();

app.get('/', function(req, res) {
    res.send('Hello World');
})

app.listen(3000, function() {
    console.log("Server started on port 3000...");
    //console.log(mySearch.response)//prints 'undefined'

    mySearch.getTweets(function(err, data, response) {
        console.log(data)
    });
})

Answer 3

When you want to use any function outside file then use module.export . to make it available.

Search.js

var Twit = require('twit');
var config = require('./config')

var T = new Twit(config);

var getTweet = function (params, callback) {
    T.get('search/tweets', params, function (err, data, response) {
        if (error) {
            console.log(error);
            callback(error,null);
        } else {
            callback(null,data);
        }
    });
}

module.exports.getTweet = getTweet;

index.js

const mySearch = require('./search.js');
const express = require('express');

const app = express();

app.get('/', function(req, res) {
    res.send('Hello World');
})

app.listen(3000, function(){
    console.log("Server started on port 3000...");
    var params = {
        q: '#stackOverflow',
        count: 1
     } 

    mySearch.getTweet(params,function(error,response){
        error ? console.log(error) : console.log(response);
    });
});

Answer 4

You have to export the module in your search.js file. The code to do so looks like this:

module.exports = {
  response: response
};

but, as stated in the comments, for a fully functional module you need to export a Promise, for example, given that fetching data from twitter is an asynchronous operation. To do so, create a function like this:

module.exports = {
  function getData() {
    return new Promise(function (resolve, reject) {
      var response = null; 

      T.get('search/tweets', params, function (err, data, response) {
        if (err) {
          reject(err);
        }
        else {
          response = data
          console.log(response) // prints the post
          resolve(response);
        }
      }); 
    });
  }
}

You can them access the retrieved tweets doing this:

mySearch.getData().then(function (response) {
  console.log(response); // Data retrieved
}).catch(function (error) {
  console.log(error); // Error!
});