How do I read files in Node.js?

Reading the contents of a file into memory is a very common programming task, and, as with many other things, the Node.js core API provides methods to make this trivial. There are a variety of file system methods, all contained in the fs module. The easiest way to read the entire contents of a file is with fs.readFile, as follows:

fs = require('fs');
fs.readFile(file, [encoding], [callback]);

// file = (string) filepath of the file to read

encoding is an optional parameter that specifies the type of encoding to read the file. Possible encodings are 'ascii', 'utf8', and 'base64'. If no encoding is provided, the default is null.

callback is a function to call when the file has been read and the contents are ready - it is passed two arguments, error and data. If there is no error, error will be null and data will contain the file contents; otherwise err contains the error message.

So if we wanted to read /etc/hosts and print it to stdout (just like Unix cat):

fs = require('fs')
fs.readFile('/etc/hosts', 'utf8', function (err,data) {
  if (err) {
    return console.log(err);
  }
  console.log(data);
});

The contents of /etc/hosts should now be visible to you, provided you have permission to read the file in the first place.

Let's now take a look at an example of what happens when you try to read an invalid file - the easiest example is one that doesn't exist.

fs = require('fs');
fs.readFile('/doesnt/exist', 'utf8', function (err,data) {
  if (err) {
    return console.log(err);
  }
  console.log(data);
});

This is the output:

{ stack: [Getter/Setter],
  arguments: undefined,
  type: undefined,
  message: 'ENOENT, No such file or directory \'/doesnt/exist\'',
  errno: 2,
  code: 'ENOENT',
  path: '/doesnt/exist' }

This is a basic Node.js Error object - it can often be useful to log err.stack directly, since this contains a stack trace to the location in code at which the Error object was created.

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