I've been trying to find a way to write to a file when using Node.js, but with no success. How can I do that?

12个回答

There are a lot of details in the filesystem API. The most common way (as far as I know) is:

var fs = require('fs');
fs.writeFile("/tmp/test", "Hey there!", function(err) {
if(err) {
return console.log(err);
}

console.log("The file was saved!");
});

lrony* Or ES2015 async/await style: await promisify(fs.writeFile)("./test.md", markdownFile);

local-host âNote that it is unsafe to use fs.writeFile multiple times on the same file without waiting for the callback. For this scenario, fs.createWriteStream is strongly recommended.â

3 年多之前 回复

3 年多之前 回复

3 年多之前 回复

7*4 This seems to write to file? What's the version to append to file?
5 年多之前 回复

6 年多之前 回复

6 年多之前 回复

?yb? Also note you can use fs.writeFileSync(...) to accomplish the same thing synchronously.

You can of course make it a little more advanced. Non-blocking, writing bits and pieces, not writing the whole file at once:

var fs = require('fs');
var stream = fs.createWriteStream("my_file.txt");
stream.once('open', function(fd) {
stream.write("My first row\n");
stream.write("My second row\n");
stream.end();
});

hurriedly% You will have to wait for it.

ℙℕℤℝ Does the 'open' event happen asynchronously? Do I have to wait for it, or can I start writing into the stream immediately?

hurriedly% I'm not sure if when the stream is flushed. My guess is that there is a possibility to flush the stream on demand.
7 年多之前 回复

7 年多之前 回复
hurriedly% You can always do a stream.end() when you've done your stream.writes(). I will add it to the example.

larry*wei When do I close the stream? Why is this non-blocking? Just curious, I am trying to write to a log file.

Currently there are three ways to write a file:

1. fs.write(fd, buffer, offset, length, position, callback)

You need to wait for the callback to ensure that the buffer is written to disk. It's not buffered.

2. fs.writeFile(filename, data, [encoding], callback)

All data must be stored at the same time; you cannot perform sequential writes.

3. fs.createWriteStream(path, [options])

Creates a WriteStream, which is convenient because you don't need to wait for a callback. But again, it's not buffered.

A WriteStream, as the name says, is a stream. A stream by definition is “a buffer” containing data which moves in one direction (source ► destination). But a writable stream is not necessarily “buffered”. A stream is “buffered” when you write n times, and at time n+1, the stream sends the buffer to the kernel (because it's full and needs to be flushed).

In other words: “A buffer” is the object. Whether or not it “is buffered” is a property of that object.

If you look at the code, the WriteStream inherits from a writable Stream object. If you pay attention, you’ll see how they flush the content; they don't have any buffering system.

If you write a string, it’s converted to a buffer, and then sent to the native layer and written to disk. When writing strings, they're not filling up any buffer. So, if you do:

write("a")
write("b")
write("c")


You're doing:

fs.write(new Buffer("a"))
fs.write(new Buffer("b"))
fs.write(new Buffer("c"))


That’s three calls to the I/O layer. Although you're using “buffers”, the data is not buffered. A buffered stream would do: fs.write(new Buffer ("abc")), one call to the I/O layer.

As of now, in Node.js v0.12 (stable version announced 02/06/2015) now supports two functions: cork() and uncork(). It seems that these functions will finally allow you to buffer/flush the write calls.

For example, in Java there are some classes that provide buffered streams (BufferedOutputStream, BufferedWriter...). If you write three bytes, these bytes will be stored in the buffer (memory) instead of doing an I/O call just for three bytes. When the buffer is full the content is flushed and saved to disk. This improves performance.

I'm not discovering anything, just remembering how a disk access should be done.

hurriedly% As of now, Node v0.12 is stable.
5 年多之前 回复
perhaps? any chance you could provide an example of how to use cork() and uncork() for those of us who want to try out the pre-release 0.11 node?

6 年多之前 回复

Synchronous Write

fs.writeFileSync(file, data[, options])

fs = require('fs');

fs.writeFileSync("synchronous.txt", "synchronous write!")


Asynchronous Write

fs.writeFile(file, data[, options], callback)

fs = require('fs');

fs.writeFile('asynchronous.txt', 'asynchronous write!', (err) => {
if (err) throw err;
console.log('The file has been saved!');
});


Where

file <string> | <Buffer> | <URL> | <integer> filename or file descriptor
data <string> | <Buffer> | <Uint8Array>
options <Object> | <string>
callback <Function>


Worth reading the offical File System (fs) docs.

 var fs = require('fs');
fs.writeFile(path + "\\message.txt", "Hello", function(err){
if (err) throw err;
console.log("success");
});


For example : read file and write to another file :

  var fs = require('fs');
var path = process.cwd();
{
if(err)
console.log(err)
else
{
fs.writeFile(path+"\\to.text",function(erro){
if(erro)
console.log("error : "+erro);
else
console.log("success");
});
}
});

Lotus＠ Where you are writting the data into the "to.text"??
3 年多之前 回复

Here we use w+ for read/write both actions and if the file path is not found the it would be created automatically.

fs.open(path, 'w+', function(err, data) {
if (err) {
console.log("ERROR !! " + err);
} else {
fs.write(data, 'content', 0, 'content length', null, function(err) {
if (err)
console.log("ERROR !! " + err);
fs.close(data, function() {
console.log('written success');
})
});
}
});


Content means what you have to write to the file and its length, 'content.length'.

Here is the sample of how to read file csv from local and write csv file to local.

var csvjson = require('csvjson'),
fs = require('fs'),
mongodb = require('mongodb'),
MongoClient = mongodb.MongoClient,
mongoDSN = 'mongodb://localhost:27017/test',
collection;

var importOptions = {
delimiter : ',', // optional
quote     : '"' // optional
},ExportOptions = {
delimiter   : ",",
wrap        : false
}
var myobj = csvjson.toSchemaObject(data, importOptions)
var exportArr = [], importArr = [];
myobj.forEach(d=>{
if(d.orderId==undefined || d.orderId=='') {
exportArr.push(d)
} else {
importArr.push(d)
}
})
var csv = csvjson.toCSV(exportArr, ExportOptions);
MongoClient.connect(mongoDSN, function(error, db) {
collection = db.collection("orders")
collection.insertMany(importArr, function(err,result){
db.close();
});
})
}



I liked Index of ./articles/file-system.

It worked for me.

fs = require('fs');
fs.writeFile('helloworld.txt', 'Hello World!', function (err) {
if (err)
return console.log(err);
console.log('Wrote Hello World in file helloworld.txt, just check it');
});


Contents of helloworld.txt:

Hello World!


Update:
As in Linux node write in current directory , it seems in some others don't, so I add this comment just in case :
Using this ROOT_APP_PATH = fs.realpathSync('.'); console.log(ROOT_APP_PATH); to get where the file is written.

Lotus＠ I just found it. Using this ROOT_APP_PATH = fs.realpathSync('.'); console.log(ROOT_APP_PATH); to get my where the file written. Thanks.

Lotus＠ That's weird... I just can't find it anywhere. Will it be hidden? thanks again~

Lotus＠ Where to find the file helloworld.txt ? I can't find it in any folders... thanks.

You can use library easy-file-manager

install first from npm npm install easy-file-manager

Sample to upload and remove files

var filemanager = require('easy-file-manager')
var path = "/public"
var filename = "test.jpg"
var data; // buffered image

if (err) console.log(err);
});

filemanager.remove(path,"aa,filename,function(isSuccess){
if (err) console.log(err);
});


I know the question asked about "write" but in a more general sense "append" might be useful in some cases as it is easy to use in a loop to add text to a file (whether the file exists or not). Use a "\n" if you want to add lines eg:

var fs = require('fs');
for (var i=0; i<10; i++){
fs.appendFileSync("junk.csv", "Line:"+i+"\n");
}

?yb? fixed, thanks 0x48piraj
2 年多之前 回复
python小菜 It should be var fs = require('fs');, Right?
2 年多之前 回复