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part1_basics_03.md

File metadata and controls

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Files

Writing

var fs = require('fs');
fs.writeFile('./file1.txt', 'Some text\n', function(err) {
  if (err) throw err;
  console.log('Wrote file1.txt');
});

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

  • writes text to a file
  • options is a hash specifying addional params like encoding. default is utf8.

Writing to a stream

var fs = require('fs');
var os = fs.createWriteStream('./file2.txt');
os.write('Some text\n');
os.write('Some text\n');
console.log('Wrote file2.txt')

fs.createWriteStream(path, [options]) (http://nodejs.org/api/fs.html#fs_fs_createwritestream_path_options)

  • opens a file for streaming and returns a new stream.Writeable

writable.write(chunk, [encoding], [callback])

  • writes a chunk to the stream

Reading

var fs = require('fs');
fs.readFile('./file2.txt', 'utf8', function(err, data) {
  if (err) throw err;
  console.log(data);
});

fs.readFile(filename, [options], callback)

  • opens a file for reading and reads all the contents
  • the callback is fired once when the reading is completed

Reading a stream

var fs = require('fs');
fs.createReadStream('./file2.txt', {encoding:'utf8',
  bufferSize:5}).
  on('data', function(chunk) {
    console.log('data:' + chunk);
  }).
  on('error', function(err) {
    throw err;
  });

fs.createReadStream(path, [options])

  • opens a file for reading and returns a new stream.Readable
  • the data event is fired for each chunk that s read.
  • error fires for any I/O related errors that occur

Streams

Streams are used when there is a continuous flow of data in/out.

  • A stream is an abstract emitter interface in node that is implemented by various objects like an HTTP Request, Files, Sockets, etc.
  • Streams wrap a file descriptor
  • node supports both readable and writable streams
  • Streams can be piped together
  • Pre node 0.10 streams are push, in node 0.10 streams are pull by default.

Readable streams

Methods

  • pause: pause a stream
  • resume: resume a paused stream
  • destroy: close the stream
  • pipe: pipe all input to another stream

Events

  • data: data has been received
  • end: there is no more data
  • error: there was an error receiving data
  • close: the underlying file descriptor has been closed

Writeable streams

Methods

  • write: write to a stream
  • end: terminate the stream
  • destroy: close the stream

###Events

  • drain: the buffer is drained
  • error: there was an error writing data
  • close: the underlying file descriptor has been closed
  • pipe: the stream has been piped

Piping

A readable stream can be piped to a writeable stream.

var https = require('https');

req = https.get('https://api.github.com/users/glennblock/repos',onGet); 

function onGet(res) {
  res.pipe(process.stdout);
}

req.on('error', function(e) {
  console.log('problem with request: ' + e.message);
});

process.stdout

Standard output stream for the process

readable.pipe(destination, [options])

  • destination: must be a writeable stream.
  • options
    • end: indicates end should be called on the destination stream. Default is true.

Other things you can do with files

  • fs.rename: rename a file
  • fs.mkdir: create a directory
  • fs.readdir: read the contents of a directory
  • fs.stat: get information about a file
  • fs.symlink: create a shortcut
  • fs.watchFile: monitor file changes