This specification addresses how modules should be written in order to be interoperable in browser-based environment. By implication, this specification defines the minimum features that a module system must provide in order to support interoperable modules.

• Modules are singletons.
• New free variables within the module scope should not be introduced.
• Execution must be lazy.

A module is defined with define keyword, which is a function.

define(factory);

1. The define function accepts a single argument, the module factory.
2. The factory may be a function or other valid values.
3. If factory is a function, the first three parameters of the function, if specified, must be "require", "exports", and "module", in that order.
4. If factory is not a function, then the module's exports are set to that object.

In a module, there are three free variables: require, exports and module.

define(function(require, exports, module) {

  // The module code goes here

});

1. require is a function

   1. require accepts a module identifier.
   2. require returns the exported API of the foreign module.
   3. If requested module cannot be returned, require should return null.

2. require.async is a function

   1. require.async accepts a list of module identifiers and a optional callback function.
   2. The callback function receives module exports as function arguments, listed in the same order as the order in the first argument.
   3. If requested module cannot be returned, the callback should receive null correspondingly.

In a module, there is a free variable called "exports", that is an object that the module may add its API to as it executes.

1. module.uri

   The full resolved uri to the module.

2. module.dependencies

   A list of module identifiers that required by the module.

3. module.exports

   The exported API of the module. It is the same as exports object.

1. A module identifier is and must be a literal string.
2. Module identifiers may not have a filename extensions like .js.
3. Module identifiers should be dash-joined string, such as foo-bar.
4. Module identifiers can be a relative path, like ./foo and ../bar.

A typical sample

math.js

define(function(require, exports, module) {
  exports.add = function() {
    var sum = 0, i = 0, args = arguments, l = args.length;
    while (i < l) {
      sum += args[i++];
    }
    return sum;
  };
});

increment.js

define(function(require, exports, module) {
  var add = require('math').add;
  exports.increment = function(val) {
    return add(val, 1);
  };
});

program.js

define(function(require, exports, module) {
  var inc = require('increment').increment;
  var a = 1;
  inc(a); // 2

  module.id == "program";
});

Wrapped modules with non-function factory

object-data.js

define({
    foo: "bar"
});

array-data.js

define([
    'foo',
    'bar'
]);

string-data.js

define('foo bar');