io.js v1.6.0 introduces a new command line option: -r or --require, which can be used to pre-load modules at startup, see:

https://github.com/iojs/io.js/blob/v1.x/CHANGELOG.md#2015-03-19-version-160-chrisdickinson

I can think of one interesting use case: overriding the default behavior or console.log to perform deep inspection:

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// console.js
var util = require('util');
var log = console.log;
console.log = function (obj) {
  log(util.inspect(obj, { colors: true, depth: null }));
};

Now I can pre-load this script to have all console log show in better format:

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$ iojs -r /path/to/console.js app.js

Time to find out more use cases.

Node has a simple module loading system by using require.

A module prefixed with '/' is an absolute path to the file. For example, require('/home/marco/foo.js') will load the file at /home/marco/foo.js.

A module prefixed with './' is relative to the file calling require(). That is, circle.js must be in the same directory as foo.js for require('./circle') to find it.

Without a leading '/' or './' to indicate a file, the module is either a “core module” or is loaded from a node_modules folder.

http://nodejs.org/api/modules.html#modules_file_modules

We can also use it to load JSON files by simply specifying the file path. File path can be relative or absolute. Giving the following file structure:

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.
├── app.js
├── data
│   └── file.json
└── lib
    └── util.js

Let’s say the root path of the application is /home/marco. Use relative path from the file lib/util.js:

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var json = require('../data/file.json');

This will load the JSON content from data/file.json, because the require system knows that the file is relative to the calling file lib/util.js. But when using file system functions, relative path is not treat the same way:

Relative path to filename can be used, remember however that this path will be relative to process.cwd(). - File System

process.cwd() returns the current working directory, not the root directory of the application nor the directory of the calling file. For example:

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var data = fs.readFileSync('../data/file.json');

If your current working directory is /home/marco/myapp, the application root directory, you will get the following error message:

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fs.js:410
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: ENOENT, no such file or directory '../data/file.json'
    at Object.fs.openSync (fs.js:410:18)
    at Object.fs.readFileSync (fs.js:276:15)

Because process.cwd() is /home/marco/myapp, and reading file relative to this directory with ../data/file.json, Node will expect file path to be /home/marco/data/file.json. Therefore, the error is thrown.

The difference between require and fs in loading files is that if the relative path is used in require, it is relative to the file calling the require. If the relative path is used in fs, it is relative to process.cwd().

To fix the error in file system functions, we should always use absolute path. One thing we can do is:

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var data = fs.readFileSync(__dirname + '/../data/file.json');

__dirname is “the name of the directory that the currently executing script is resides in” [dirname], and it has no trailing slash. The the structure looks odd, even though it works, but it might not work in all systems.

The correct solution here is to use path.join from Path:

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var data = fs.readFileSync(path.join(__dirname, '../data/file.json'));

The path module “contains utilities for handling and transforming file paths”. The path.join function will call path.normalize to “normalize a string path, taking care of .. and . paths” [normalize], also see function normalizeArray in https://github.com/joyent/node/blob/master/lib/path.js on how the paths are normalized.

In conclusion:

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// Use `require` to load JSON file.
var json = require('../data/file.json');
// Use `fs` to load JSON file.
var data = fs.readFileSync(path.join(__dirname, '../data/file.json'));
var json = JSON.parse(data.toString());