There are a lot of details in the File System API. The most common way is:

const fs = require( fs );

fs.writeFile("/tmp/test", "Hey there!", function(err) {
    if(err) {
        return console.log(err);
    }
    console.log("The file was saved!");
}); 

// Or
fs.writeFileSync( /tmp/test-sync ,  Hey there! );

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.

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
");
  stream.write("My second row
");
  stream.end();
});

Synchronous Write

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

fs = require( fs );

fs.writeFileSync("foo.txt", "bar");

Asynchronous Write

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

fs = require( fs );

fs.writeFile( foo.txt ,  bar , (err) => { if (err) throw err; });

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.

Update: async/await

fs = require( fs );
util = require( util );
writeFile = util.promisify(fs.writeFile);

fn = async () => { await writeFile( foo.txt ,  bar ); }

fn()

var path =  public/uploads/file.txt ,
buffer = new Buffer("some content
");

fs.open(path,  w , function(err, fd) {
    if (err) {
        throw  error opening file:   + err;
    }

    fs.write(fd, buffer, 0, buffer.length, null, function(err) {
        if (err) throw  error writing file:   + err;
        fs.close(fd, function() {
            console.log( file written );
        })
    });
});

The answers provided are dated and a newer way to do this is:

const fsPromises = require( fs ).promises
await fsPromises.writeFile( /path/to/file.txt ,  data to write )

see documents here for more info

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

It worked for me.

See also How do I write files in node.js?.

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.

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 " " if you want to add lines eg:

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

OK, it s quite simple as Node has built-in functionality for this, it s called fs which stands for File System and basically, NodeJS File System module...

So first require it in your server.js file like this:

var fs = require( fs );

fs has few methods to do write to file, but my preferred way is using appendFile, this will append the stuff to the file and if the file doesn t exist, will create one, the code could be like below:

fs.appendFile( myFile.txt ,  Hi Ali! , function (err) {
  if (err) throw err;
  console.log( Thanks, It s saved to the file! );
});

You may write to a file using fs (file system) module.

Here is an example of how you may do it:

const fs = require( fs );

const writeToFile = (fileName, callback) => {
  fs.open(fileName,  wx , (error, fileDescriptor) => {
    if (!error && fileDescriptor) {
      // Do something with the file here ...
      fs.writeFile(fileDescriptor, newData, (error) => {
        if (!error) {
          fs.close(fileDescriptor, (error) => {
            if (!error) {
              callback(false);
            } else {
              callback( Error closing the file );
            }
          });
        } else {
          callback( Error writing to new file );
        }
      });
    } else {
      callback( Could not create new file, it may already exists );
    }
  });
};

You might also want to get rid of this callback-inside-callback code structure by useing Promises and async/await statements. This will make asynchronous code structure much more flat. For doing that there is a handy util.promisify(original) function might be utilized. It allows us to switch from callbacks to promises. Take a look at the example with fs functions below:

// Dependencies.
const util = require( util );
const fs = require( fs );

// Promisify "error-back" functions.
const fsOpen = util.promisify(fs.open);
const fsWrite = util.promisify(fs.writeFile);
const fsClose = util.promisify(fs.close);

// Now we may create  async  function with  await s.
async function doSomethingWithFile(fileName) {
  const fileDescriptor = await fsOpen(fileName,  wx );

  // Do something with the file here...

  await fsWrite(fileDescriptor, newData);
  await fsClose(fileDescriptor);
}

You can write to files with streams.

Just do it like this:

const fs = require( fs );

const stream = fs.createWriteStream( ./test.txt );
stream.write("Example text");

 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();
    fs.readFile(path+"\from.txt",function(err,data)
                {
                    if(err)
                        console.log(err)
                    else
                        {
                            fs.writeFile(path+"\to.text",function(erro){
                                if(erro)
                                    console.log("error : "+erro);
                                else
                                    console.log("success");
                            });
                        }
                });

Here we use w+ for read/write both actions and if the file path is not found then 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;

function uploadcsvModule(){
    var data = fs.readFileSync(  /home/limitless/Downloads/orders_sample.csv , { encoding :  utf8 });
    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){
            fs.writeFile( /home/limitless/Downloads/orders_sample1.csv , csv, { encoding :  utf8 });
            db.close();
        });            
    })
}

uploadcsvModule()

fs.createWriteStream(path[,options])

options may also include a start option to allow writing data at some position past the beginning of the file. Modifying a file rather than replacing it may require a flags mode of r+ rather than the default mode w. The encoding can be any one of those accepted by Buffer.

If autoClose is set to true (default behavior) on error or finish the file descriptor will be closed automatically. If autoClose is false, then the file descriptor won t be closed, even if there s an error. It is the application s responsibility to close it and make sure there s no file descriptor leak.

Like ReadStream, if fd is specified, WriteStream will ignore the path argument and will use the specified file descriptor. This means that no open event will be emitted. fd should be blocking; non-blocking fds should be passed to net.Socket.

If options is a string, then it specifies the encoding.

After, reading this long article. You should understand how it works. So, here s an example of createWriteStream().

/* The fs.createWriteStream() returns an (WritableStream {aka} internal.Writeable) and we want the encoding as  utf -8 */
/* The WriteableStream has the method write() */
fs.createWriteStream( out.txt ,  utf-8 )
.write( hello world );

Point 1:

If you want to write something into a file. means: it will remove anything already saved in the file and write the new content. use fs.promises.writeFile()

Point 2:

If you want to append something into a file. means: it will not remove anything already saved in the file but append the new item in the file content.then first read the file, and then add the content into the readable value, then write it to the file. so use fs.promises.readFile and fs.promises.writeFile()

example 1: I want to write a JSON object in my JSON file .

const fs = require( fs );
const data = {table:[{id: 1, name:  my name }]}
const file_path =  ./my_data.json 
writeFile(file_path, data)
async function writeFile(filename, writedata) {
  try {
    await fs.promises.writeFile(filename, JSON.stringify(writedata, null, 4),  utf8 );
    console.log( data is written successfully in the file )
  }
  catch (err) {
    console.log( not able to write data in the file  )
  }
}

example2 : if you want to append data to a JSON file. you want to add data {id:1, name: my name } to file my_data.json on the same folder root. just call append_data (file_path , data ) function.

It will append data in the JSON file if the file existed . or it will create the file and add the data to it.

const fs = require( fs );
const data = {id: 2, name:  your name }
const file_path =  ./my_data.json 
append_data(file_path, data)

async function append_data(filename, data) {

  if (fs.existsSync(filename)) {
    var read_data = await readFile(filename)
    if (read_data == false) {
      console.log( not able to read file )
    } else {
      read_data.table.push(data)  //data must have the table array in it like example 1
      var dataWrittenStatus = await writeFile(filename, read_data)
      if (dataWrittenStatus == true) {
        console.log( data added successfully )
      } else {
        console.log( data adding failed )
      }
    }
  }
}

async function readFile(filePath) {
  try {
    const data = await fs.promises.readFile(filePath,  utf8 )
    return JSON.parse(data)
  }
  catch (err) {
    return false;
  }
}

async function writeFile(filename, writedata) {
  try {
    await fs.promises.writeFile(filename, JSON.stringify(writedata, null, 4),  utf8 );
    return true
  }
  catch (err) {
    return false
  }
}

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

filemanager.upload(path,filename,data,function(err){
    if (err) console.log(err);
});

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

You can write in a file by the following code example:

var data = [{  test :  123 ,  test2 :  Lorem Ipsem   }];
fs.open(datapath +  /data/topplayers.json ,  wx , function (error, fileDescriptor) {
  if (!error && fileDescriptor) {
    var stringData = JSON.stringify(data);
    fs.writeFile(fileDescriptor, stringData, function (error) {
      if (!error) {
        fs.close(fileDescriptor, function (error) {
          if (!error) {
            callback(false);
          } else {
            callback( Error in close file );
          }
        });
      } else {
        callback( Error in writing file. );
      }
    });
  }
});