I am trying to append a string to a log file. However writeFile will erase the content each time before writing the string.
fs.writeFile(\'log.txt\'
For occasional appends, you can use appendFile
, which creates a new file handle each time it's called:
Asynchronously:
const fs = require('fs');
fs.appendFile('message.txt', 'data to append', function (err) {
if (err) throw err;
console.log('Saved!');
});
Synchronously:
const fs = require('fs');
fs.appendFileSync('message.txt', 'data to append');
But if you append repeatedly to the same file, it's much better to reuse the file handle.
My approach is rather special. I basically use the WriteStream
solution but without actually 'closing' the fd by using stream.end()
. Instead I use cork
/uncork
. This got the benefit of low RAM usage (if that matters to anyone) and I believe it's more safe to use for logging/recording (my original use case).
Following is a pretty simple example. Notice I just added a pseudo for
loop for showcase -- in production code I am waiting for websocket messages.
var stream = fs.createWriteStream("log.txt", {flags:'a'});
for(true) {
stream.cork();
stream.write("some content to log");
process.nextTick(() => stream.uncork());
}
uncork
will flush the data to the file in the next tick.
In my scenario there are peaks of up to ~200 writes per second in various sizes. During night time however only a handful writes per minute are needed. The code is working super reliable even during peak times.
fd = fs.openSync(path.join(process.cwd(), 'log.txt'), 'a')
fs.writeSync(fd, 'contents to append')
fs.closeSync(fd)
If you want an easy and stress-free way to write logs line by line in a file, then I recommend fs-extra:
const os = require('os');
const fs = require('fs-extra');
const file = 'logfile.txt';
const options = {flag: 'a'};
async function writeToFile(text) {
await fs.outputFile(file, `${text}${os.EOL}`, options);
}
writeToFile('First line');
writeToFile('Second line');
writeToFile('Third line');
writeToFile('Fourth line');
writeToFile('Fifth line');
Tested with Node v8.9.4.
Using fs.appendFile
or fsPromises.appendFile
are the fastest and the most robust options when you need to append something to a file.
In contrast to some of the answers suggested, if the file path is supplied to the appendFile
function, It actually closes by itself. Only when you pass in a filehandle that you get by something like fs.open()
you have to take care of closing it.
I tried it with over 50,000 lines in a file.
Examples :
(async () => {
// using appendFile.
const fsp = require('fs').promises;
await fsp.appendFile(
'/path/to/file', '\r\nHello world.'
);
// using apickfs; handles error and edge cases better.
const apickFileStorage = require('apickfs');
await apickFileStorage.writeLines(
'/path/to/directory/', 'filename', 'Hello world.'
);
})();
Ref: https://github.com/nodejs/node/issues/7560
Example Execution: https://github.com/apickjs/apickFS/blob/master/writeLines.js
const inovioLogger = (logger = "") => {
const log_file = fs.createWriteStream(__dirname + `/../../inoviopay-${new Date().toISOString().slice(0, 10)}.log`, { flags: 'a' });
const log_stdout = process.stdout;
log_file.write(logger + '\n');
}