node - fs - reading two files so they can be compared

Question

How can I read two file using fs and have both result available in a way , where I can compare 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

try `fs.readFileSync`, the "Synchronous version of `fs.readFile()`. Returns the contents of the path." — Kelvin Sherlock, Feb 06 '18 at 15:54
If you can: https://github.com/jprichardson/node-fs-extra gives a version of `readFile` returning a promise instead of using a callback. Otherwise look at [util.promisify](https://nodejs.org/dist/latest-v8.x/docs/api/util.html#util_util_promisify_original). — Josh Lee, Feb 06 '18 at 15:56

Daniel A. White · Accepted Answer · 2018-02-06T15:55:26.243

5

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];
});

edited Feb 06 '18 at 15:55

answered Feb 06 '18 at 15:52

Daniel A. White

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when I do like above plus this code `var fileDiff = require('diff'); var difference = fileDiff.diffSentences(file1, file2);` or `diffChars` , I get 'TypeError: value.split is not a function' – user1207289 Feb 06 '18 at 16:53
so why does `console.log(typeof file1)` and `console.log(typeof file2)` returns a `string` ? – user1207289 Feb 06 '18 at 17:00
it worked after I added `.toString()` to `file1` and `file2` . Thanks! – user1207289 Feb 06 '18 at 17:15

Moritz Schmitz v. Hülst · Answer 2 · 2018-02-06T16:01:06.233

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);
  });
});

edited Feb 06 '18 at 16:01

answered Feb 06 '18 at 15:57

Moritz Schmitz v. Hülst

3,229
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1

`readFileSync` is blocking and will block the node event loop. In other words, doing this will stop everything running until the files are read. – Jim Wright Feb 06 '18 at 15:59
@JimWright which *could* be ok, depending on the context of usage. I know it's definitely not near as common as an async read though, but valid contexts exist for sure. – Sterling Archer Feb 06 '18 at 16:00
@JimWright, edited my answer, thanks. It does make sense to mention it. – Moritz Schmitz v. Hülst Feb 06 '18 at 16:01
@Moritz Schmitz v. Hülst same as my comment above on Daniel A. White answer – user1207289 Feb 06 '18 at 17:03

score 0 · Answer 3 · answered Feb 06 '18 at 16:02

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;})

node - fs - reading two files so they can be compared

3 Answers3