How can I read two file using fs and have both result available in a way , where I can pare them. I looked at this but its slightly different and I couldnt find a way to do what I need.

I can call the diffChars from callback, but how to do with two callback functions?

  fs.readFile('/abc1.txt',  function (err, data1) {
    console.log(data1);
});
  fs.readFile('/abc1.txt',  function (err, data2) {
    console.log(data2);
});

 later I want to do like this

 var fileDiff = require("diff");
 var difference = fileDiff.diffChars(data1,data2);
 cnsole.log(difference);

note: I am restrictive on libraries I can use because of npm proxy repository

How can I read two file using fs and have both result available in a way , where I can pare them. I looked at this but its slightly different and I couldnt find a way to do what I need.

I can call the diffChars from callback, but how to do with two callback functions?

  fs.readFile('/abc1.txt',  function (err, data1) {
    console.log(data1);
});
  fs.readFile('/abc1.txt',  function (err, data2) {
    console.log(data2);
});

 later I want to do like this

 var fileDiff = require("diff");
 var difference = fileDiff.diffChars(data1,data2);
 cnsole.log(difference);

note: I am restrictive on libraries I can use because of npm proxy repository

• javascript
• node.js

• try fs.readFileSync, the "Synchronous version of fs.readFile(). Returns the contents of the path." – Kelvin Sherlock Commented Feb 6, 2018 at 15:54
• 1 If you can: github./jprichardson/node-fs-extra gives a version of readFile returning a promise instead of using a callback. Otherwise look at util.promisify. – Josh Lee Commented Feb 6, 2018 at 15:56

3 Answers 3

This is a perfect case for a Promise and Promise.all.

function readFile(name) {
    return new Promise((resolve, reject) =>
        fs.readFile(name,  function (err, data) {
            if (err) { reject(err); }
            resolve(data);
        });
    });
}

Promise.all(readFile('file1'), readFile('file2')).then(data => {
   var file1 = data[0];
   var file2 = data[1];
});

You don't need the callback functions. You could use fs.readFileSync().

EDIT However, fs.readFileSync() is blocking and the next line will only be executed once the function returns.

If you insist on using callbacks:

fs.readFile("abc123.txt", (error1, data1) => {
  if (error1) {
    return;
  }

  fs.readFile("abc456.txt", (error2, data2) => {
    if (error2) {
      return;
    }

    console.log(data1 === data2);
  });
});

Node uses asynchronous style of coding. So when you are reading file one the 2nd file is also being read and also the next code is being executed. Thus you can use promises or callbacks. Here is one solution

fs.readFile('/abc1.txt',  function (err, data1) {
    console.log(data1);
    fs.readFile('/abc1.txt',  function (err, data2) {
        console.log(data2);
        var fileDiff = require("diff");
        var difference = fileDiff.diffChars(data1,data2);
        console.log(difference);
    });      
});

or use promises like

var data;
fs.readFile('/abc1.txt')
.then((data1)=>{
    data = data1;
    return(fs.readFile('/abc1.txt'))
})
.then((data2)=>{
    var difference = fileDiff.diffChars(data1,data2);
    return(difference)
})
/* do what ever you want here*/
.catch((err)=>{throw err;})