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', 'Hello Node', function (err) {
if (err) throw err;
console.log('It\'s saved!');
}); // => message.txt erased, contains only 'Hello Node'
Any idea how to do this the easy way?
Your code using createWriteStream creates a file descriptor for every write. log.end is better because it asks node to close immediately after the write.
var fs = require('fs');
var logStream = fs.createWriteStream('log.txt', {flags: 'a'});
// use {flags: 'a'} to append and {flags: 'w'} to erase and write a new file
logStream.write('Initial line...');
logStream.end('this is the end line');
Use a+ flag to append and create a file (if doesn't exist):
fs.writeFile('log.txt', 'Hello Node', { flag: "a+" }, (err) => {
if (err) throw err;
console.log('The file is created if not existing!!');
});
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.'
);
})();
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.
Try to use flags: 'a' to append data to a file
var stream = fs.createWriteStream("udp-stream.log", {'flags': 'a'});
stream.once('open', function(fd) {
stream.write(msg+"\r\n");
});
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');
}
In addition to denysonique's answer, sometimes asynchronous type of appendFile and other async methods in NodeJS are used where promise returns instead of callback passing. To do it you need to wrap the function with promisify HOF or import async functions from promises namespace:
const { appendFile } = require('fs').promises;
await appendFile('path/to/file/to/append', dataToAppend, optionalOptions);
I hope it'll help
I wrapped the async fs.appendFile into a Promise-based function. Hope it helps others to see how this would work.
append (path, name, data) {
return new Promise(async (resolve, reject) => {
try {
fs.appendFile((path + name), data, async (err) => {
if (!err) {
return resolve((path + name));
} else {
return reject(err);
}
});
} catch (err) {
return reject(err);
}
});
}