Question

The following is a minimal implementation of require :

function require(name) {
  var code = new Function("exports", readFile(name));
  var exports = {};
  code(exports);
  return exports;
}

console.log(require("weekDay").name(1));
// → Monday

Since the new Function constructor wraps the module code in a function, we don’t have to write a wrapping namespace function in the module file itself. And since we make exports an argument to the module function, the module does not have to declare it. This removes a lot of clutter from our example module.

var names = ["Sunday", "Monday", "Tuesday", "Wednesday",
             "Thursday", "Friday", "Saturday"];

exports.name = function(number) {
  return names[number];
};
exports.number = function(name) {
  return names.indexOf(name);
};

When using this pattern, a module typically starts with a few variable declarations that load the modules it depends on.

var weekDay = require("weekDay");
var today = require("today");

console.log(weekDay.name(today.dayNumber()));

The simplistic implementation of require given previously has several problems. For one, it will load and run a module every time it is require d, so if several modules have the same dependency or a require call is put inside a function that will be called multiple times, time and energy will be wasted.

This can be solved by storing the modules that have already been loaded in an object and simply returning the existing value when one is loaded multiple times.

The second problem is that it is not possible for a module to directly export a value other than the exports object, such as a function. For example, a module might want to export only the constructor of the object type it defines. Right now, it cannot do that because require always uses the exports object it creates as the exported value.

The traditional solution for this is to provide modules with another variable, module , which is an object that has a property exports . This property initially points at the empty object created by require but can be overwritten with another value in order to export something else.

function require(name) {
  if (name in require.cache)
    return require.cache[name];

  var code = new Function("exports, module", readFile(name));
  var exports = {}, module = {exports: exports};
  code(exports, module);

  require.cache[name] = module.exports;
  return module.exports;
}
require.cache = Object.create(null);

We now have a module system that uses a single global variable ( require ) to allow modules to find and use each other without going through the global scope.

This style of module system is called CommonJS modules, after the pseudo-standard that first specified it. It is built into the Node.js system. Real implementations do a lot more than the example I showed. Most importantly, they have a much more intelligent way of going from a module name to an actual piece of code, allowing both pathnames relative to the current file and module names that point directly to locally installed modules.

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.