Singleton JavaScript modules that are easy to test! (Hint: Avoid object literals)

gistfile1.js

// using AMD syntax here to avoid confusion about scope and file boundaries,
// but the concept translates 1:1 to CommonJS (node/io) modules

define('badSingleton', function(require, exports, module) {
    var dependency = require('dependency');

    var privateState;

    module.exports = {
        foo: function() {
            return privateState;
        },
        bar: function() {
            privateState = dependency(privateState ? 'bar': 'baz');
        },
        baz: function() {
            if (privateState) {
                return Promise.resolve(privateState);
            }

            return Promise.reject();
        }
    }
});

// once you call .bar, there is no way to reset the internal state of badSingleton.
// The order of your unit tests now matters!
// also, mocking dependency is non-trivial in both node and the browser

define('betterSingleton', function(require, exports, module) {
    var dependency = require('dependency');

    function BetterSingleton(dependency) {
        // move ALL the state inside the constructor
        var privateState;
        
        this.foo = function() {
            return privateState;
        };
        this.bar = function() {
            privateState = dependency(privateState ? 'bar': 'baz');
        };
        this.baz = function() {
            if (privateState) {
                return Promise.resolve(privateState);
            }

            return Promise.reject();
        }
    }

    

    module.exports = new BetterSingleton(dependency);
});

// now in your tests you can write
// var BetterSingletonConstructor = require('betterSingleton').constructor;
// and create as many instances as you have tests, each starting with a clean state.
// and since we moved the dependency into a constructor argument
// the tests can make it with ease if needed