fs.readFileSync always returns empty string

Question

I have a script that writes data from an API to some files. I have an object the contains the file descriptors for each file:

var csvFds = {
    'file1' : null,
    'file2' : null,
    'file3' : null,
    'file4' : null
};

for (var file in csvFds) {

    var dirPath = __dirname + '/files/' + file;

    try {
        fs.statSync(dirPath);
    }
    catch (e) {
        mkdirp.sync(dirPath, {mode: 0755});
    }

    csvFds[file] = fs.openSync(dirPath + '/' + moment().format("YYYY-MM-DDTHH:mm:ss[Z]") + '.csv', 'a+');
}

Then I have some code that uses fs.write to write lines of csv to the file in batches. This part is working fine. I have well formed csv files. Now I need to read the contents of the entire file as a string. This is how I'm trying to do it:

fs.readFileSync(csvFds['file1']).toString();

But for some reason I am always getting an empty string. I have confirmed that fs.readFileSync is in fact returning a Buffer by using console.log and dropping the toString() method.

I'm really stuck on this so any help will be greatly appreciated. Thanks in advance. Here's some additional info regarding my node version and OS:

$ node -v
v6.2.3-pre

$ uname -a
Darwin i-2.local 14.5.0 Darwin Kernel Version 14.5.0: Thu Jun 16 19:58:21 PDT 2016; root:xnu-2782.50.4~1/RELEASE_X86_64 x86_64

if you provide encoding to readFileSync method you will get a string instead of a Buffer — mic4ael
– mic4ael, Commented Aug 9, 2016 at 20:15
I've just discovered that a file with [], as in an empty array serialized, is read with readFileSync it returns an empty string and length=0. I've tried returning as buffer and inspecting, it's the same. on macOS + Node v16.16. — Janaka
– Janaka, Commented Jul 31, 2022 at 11:17

bormat · Accepted Answer · 2019-05-01 16:19:54Z

4

For others who have the same problem.

For me, the only way to have a non-empty string was to use fs.readFile instead of fs.readFileSync.

I use a Mac and I was trying to read a file that node create itself. If I try to read another file it works.

fs.readFile(file, (err, data)=>{
    if(err){
        console.log(err)
        throw err
    }else{
        let file_content = data.toString('utf8')
        // your code here
    }
})

answered May 1, 2019 at 16:19

bormat

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5 Comments

Panu Logic Over a year ago

I have the same problem sometimes even with async fs.readFile(). The data is sometimes "" even though I can verify that the file exists and has non-empty content by then doing a sync read in debugger. This happens on Windows 10.

bormat Over a year ago

@Panu Logic are you reading a file that is used by another stream or program?

Panu Logic Over a year ago

Could be used by the editor I'm editing it with. Not sure. But I found a get-around. If the content seems to be empty I do a sync-read in my callback function now and I get the correct content always so far it seems

bormat Over a year ago

ho ok, interesting.

Gorzas Over a year ago

This solution works for me, same situation (files created with NodeJS 12) but I don't why it happens. Does somebody have a clue?

mic4ael · Accepted Answer · 2016-08-09 20:18:13Z

1

Try to call readFileSync like this readFileSync(csvFds['file1'], 'utf-8'). It ought to return a string. Or you can omit the argument and then provide the encoding when calling toString method e.g. readFileSync(csvFds['file1']).toString('utf-8')

answered Aug 9, 2016 at 20:18

mic4ael

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12 Comments

Vicky Vargas Over a year ago

Hey Michael. I had actually tried both of these things previously and just did it again. But I still get an empty string. There must be some way to get information about the file descriptors or Buffers to debug this. I'm about to try switching the node version.

mic4ael Over a year ago

are you sure your csvFds dictionary contains correct data? can you show what csvFds contains after for loop ends?

mic4ael Over a year ago

it is a wild guess but this piece of code now.getTime() seems a bit suspicious to me

Vicky Vargas Over a year ago

Here's is what csvFds contains just before calling readFileSync: { file1: 13, file2: 14, file3: 15, file4: 16 } My version of the code is actually using moment.js to format the date not now.getTime(). I think the file descriptors are fine after the initial loop because I am able to write multiple batches of data to these files without any issues.

Vicky Vargas Over a year ago

It's a super simple csv file like this but it's a few thousand lines long each: 168722|admin 328701|developer 172090|mail 328702|team Not sure why the line breaks don't work

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