Singleton Tests

singleton_test.js

var assert = require('assert');

describe('how not to make a singleton', function () {

  var a, b;

  beforeEach(function () {
    a = { message: "Hello, World!" };
    b = { message: "Hello, World!" };
  });

  it('says matching strings are equal', function () {
    assert.ok(a.message === b.message);
  });

  it('says different objects are different', function () {
    assert.equal(a === b, false);
  });

});


describe('how to make a singleton', function () {

  function Builder() {
    this.instance = null;
  }

  Builder.prototype.build = function () {
    if (!this.instance) {
      this.instance = { message: "Hello, World!" };
    }
    return this.instance;
  };

  it('creates the same instance with a builder', function () {
    var builder = new Builder();
    var a = builder.build();
    var b = builder.build();
    assert.equal(a === b, true);
  });

  it('does not create the same instance if you make two builders', function () {
    var a = new Builder().build();
    var b = new Builder().build();
    assert.equal(a === b, false);
  });

  it('modifies the two objects together', function () {
    var builder = new Builder();
    var a = builder.build();
    var b = builder.build();
    a.value = 123;
    assert.equal(b.value, 123);
  });

});