How to handle circular dependencies with RequireJS/AMD?

In my system, I have a number of "classes" loaded in the browser each a separate files during development, and concatenated together for production. As they are loaded, they initialize a property on a global object, here G, as in this example:

var G = {};

G.Employee = function(name) {
    this.name = name;
    this.company = new G.Company(name + "'s own company");
};

G.Company = function(name) {
    this.name = name;
    this.employees = [];
};
G.Company.prototype.addEmployee = function(name) {
    var employee = new G.Employee(name);
    this.employees.push(employee);
    employee.company = this;
};

var john = new G.Employee("John");
var bigCorp = new G.Company("Big Corp");
bigCorp.addEmployee("Mary");

Instead of using my own global object, I am considering to make each class its own AMD module, based on James Burke's suggestion:

define("Employee", ["Company"], function(Company) {
    return function (name) {
        this.name = name;
        this.company = new Company(name + "'s own company");
    };
});
define("Company", ["Employee"], function(Employee) {
    function Company(name) {
        this.name = name;
        this.employees = [];
    };
    Company.prototype.addEmployee = function(name) {
        var employee = new Employee(name);
        this.employees.push(employee);
        employee.company = this;
    };
    return Company;
});
define("main", ["Employee", "Company"], function (Employee, Company) {
    var john = new Employee("John");
    var bigCorp = new Company("Big Corp");
    bigCorp.addEmployee("Mary");
});

The issue is that before, there was no declare-time dependency between Employee and Company: you could put the declaration in whatever order you wanted, but now, using RequireJS, this introduces a dependency, which is here (intentionally) circular, so the above code fails. Of course, in addEmployee(), adding a first line var Employee = require("Employee"); would make it work, but I see this solution as inferior to not using RequireJS/AMD as it requires me, the developer, to be aware of this newly created circular dependency and do something about it.

Is there a better way to solve this problem with RequireJS/AMD, or am I using RequireJS/AMD for something it was not designed for?

Solution 1:

This is indeed a restriction in the AMD format. You could use exports, and that problem goes away. I find exports to be ugly, but it is how regular CommonJS modules solve the problem:

define("Employee", ["exports", "Company"], function(exports, Company) {
    function Employee(name) {
        this.name = name;
        this.company = new Company.Company(name + "'s own company");
    };
    exports.Employee = Employee;
});
define("Company", ["exports", "Employee"], function(exports, Employee) {
    function Company(name) {
        this.name = name;
        this.employees = [];
    };
    Company.prototype.addEmployee = function(name) {
        var employee = new Employee.Employee(name);
        this.employees.push(employee);
        employee.company = this;
    };
    exports.Company = Company;
});

Otherwise, the require("Employee") you mention in your message would work too.

In general with modules you need to be more aware of circular dependencies, AMD or not. Even in plain JavaScript, you have to be sure to use an object like the G object in your example.

Solution 2:

I think this is quite a drawback in larger projects where (multi-level) circular dependencies dwell undetected. However, with madge you can print a list of circular dependencies to approach them.

madge --circular --format amd /path/src

Solution 3:

If you don't need your dependencies to be loaded at the start (e.g., when you are extending a class), then this is what you can do: (taken from http://requirejs.org/docs/api.html#circular)

In the file a.js:

    define( [ 'B' ], function( B ){

        // Just an example
        return B.extend({
            // ...
        })

    });

And in the other file b.js:

    define( [ ], function( ){ // Note that A is not listed

        var a;
        require(['A'], function( A ){
            a = new A();
        });

        return function(){
            functionThatDependsOnA: function(){
                // Note that 'a' is not used until here
                a.doStuff();
            }
        };

    });

In the OP's example, this is how it would change:

    define("Employee", [], function() {

        var Company;
        require(["Company"], function( C ){
            // Delayed loading
            Company = C;
        });

        return function (name) {
            this.name = name;
            this.company = new Company(name + "'s own company");
        };
    });

    define("Company", ["Employee"], function(Employee) {
        function Company(name) {
            this.name = name;
            this.employees = [];
        };
        Company.prototype.addEmployee = function(name) {
            var employee = new Employee(name);
            this.employees.push(employee);
            employee.company = this;
        };
        return Company;
    });

    define("main", ["Employee", "Company"], function (Employee, Company) {
        var john = new Employee("John");
        var bigCorp = new Company("Big Corp");
        bigCorp.addEmployee("Mary");
    });

Solution 4:

I looked at the docs on circular dependencies :http://requirejs.org/docs/api.html#circular

If there is a circular dependency with a and b , it says in your module to add require as a dependency in your module like so :

define(["require", "a"],function(require, a) { ....

then when you need "a" just call "a" like so:

return function(title) {
        return require("a").doSomething();
    }

This worked for me