Node.js my function showing undefined dictionary value despite assigning it-CodePudding

I'm new to javascript so i'm sorry if this has been asked before.

I have this function which i use to read json files.

const jsonfile = require('jsonfile');

function readData(fileName: string, callback) {
jsonfile.readFile(".\\data\\"   fileName   ".json", 'utf8', function (err, data) {
    if (err) console.error(err)
    callback(null, data);
 })
}

Then I call it using this:

// Read JSON files
var fileList = ["guides"];
var files = {};

for (let i = 0; i < fileList.length; i  ) {
    readData(fileList[i], function (err, result) {
        if (err) throw err;
        files[i] = result;
    })
}

but when I:

console.log(files['guides']);

it returns undefined. Can anyone help me fix this? Thank you very much.

CodePudding user response：

Callback function of the jsonfile.readFile function is called asynchronously and your console.log statement is executed synchronously.

Asynchronous code is executed after the synchronous execution of your javascript code has ended; as a result, your console.log statement is logging files['guides'] before guides property is added in the files object. This is why you get undefined.

Following code example shows this problem in action:

let a = 1;

setTimeout(() => {
  a = 2; 
}, 100);

console.log(a);

Above code snippet outputs 1 because the callback function of setTimeout is invoked asynchronously, i.e. after the console.log(a) statement has been executed and at the time of execution of console.log statement, value of a is 1.

Solution

To make sure that you log files["guides"] after all the files have been read and files object has been populated, you could return a promise from the readData function.

Following code shows how you could create a promise wrapper around readData function:

function readData(fileName) {
   return new Promise((resolve, reject) => {
      jsonfile.readFile(".\\data\\"   fileName   ".json", 'utf8', function (err, data) {
         if (err) reject(err);
         else resolve(data);
      })
   }
};

Now you can call the above function as:

Promise.all(fileList.map(filePath => readData(filePath)))
  .then(dataFromAllFiles => {
     console.log(dataFromAllFiles);
  })
  .catch(error => console.log(error));

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