Question

One of the most commonly used built-in modules that comes with Node is the "fs" module, which stands for file system. This module provides functions for working with files and directories.

For example, there is a function called readFile , which reads a file and then calls a callback with the file’s contents.

var fs = require("fs");
fs.readFile("file.txt", "utf8", function(error, text) {
  if (error)
    throw error;
  console.log("The file contained:", text);
});

The second argument to readFile indicates the character encoding used to decode the file into a string. There are several ways in which text can be encoded to binary data, but most modern systems use UTF-8 to encode text, so unless you have reasons to believe another encoding is used, passing "utf8" when reading a text file is a safe bet. If you do not pass an encoding, Node will assume you are interested in the binary data and will give you a Buffer object instead of a string. This is an array-like object that contains numbers representing the bytes in the files.

var fs = require("fs");
fs.readFile("file.txt", function(error, buffer) {
  if (error)
    throw error;
  console.log("The file contained", buffer.length, "bytes.",
              "The first byte is:", buffer[0]);
});

A similar function, writeFile , is used to write a file to disk.

var fs = require("fs");
fs.writeFile("graffiti.txt", "Node was here", function(err) {
  if (err)
    console.log("Failed to write file:", err);
  else
    console.log("File written.");
});

Here, it was not necessary to specify the encoding since writeFile will assume that if it is given a string to write, rather than a Buffer object, it should write it out as text using its default character encoding, which is UTF-8.

The "fs" module contains many other useful functions: readdir will return the files in a directory as an array of strings, stat will retrieve information about a file, rename will rename a file, unlink will remove one, and so on. See the documentation at nodejs.org for specifics.

Many of the functions in "fs" come in both synchronous and asynchronous variants. For example, there is a synchronous version of readFile called readFileSync .

var fs = require("fs");
console.log(fs.readFileSync("file.txt", "utf8"));

Synchronous functions require less ceremony to use and can be useful in simple scripts, where the extra speed provided by asynchronous I/O is irrelevant. But note that while such a synchronous operation is being performed, your program will be stopped entirely. If it should be responding to the user or to other machines on the network, being stuck on synchronous I/O might produce annoying delays.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.