Characterization tests using Mocha and expect.js to understand how functional references in JavaScript work

javascript_function_reference_characterization_spec.js

var expect = require("expect.js");

// Characterization tests using Mocha and expect.js to understand how
// functional references in JavaScript work
// See http://comp-phil.blogspot.com/2014/02/fun-with-function-references-in.html

// examples based on https://leanpub.com/javascript-allonge/read#recursive

describe("Let's play around with function references", function(){
  describe("Simple function examples", function(){
    var alwaysOne;
    beforeEach(function(done){
      alwaysOne = function(){return 1;};
      done();
    });
    it("Given a function with two references with different names " +
        "they will be equal", function(){
      var one = alwaysOne;
      expect(alwaysOne).to.be.equal(one);
    }),
    it("Given a function with two references with different names " +
       "the results will be equal", function(){
      var one = alwaysOne;
      expect(alwaysOne()).to.be.equal(one());
    }),
    it("Given a function with two references with different names " +
       "changes to the reference of one will not affect the other", function(){
      var one = alwaysOne;
      expect(alwaysOne()).to.be.equal(one());
      alwaysOne = void 0; // always is not forever :)
      expect(alwaysOne).to.not.be.equal(one);
      expect(alwaysOne).to.be.eql(void 0);
      expect(one()).to.be.equal(1);
    });
  }),
  describe("Recursive function examples", function(){
    var even;
    beforeEach(function(done){
      even = function(x){return x === 0 || !even(x-1);};
      done();
    });
    it("Given a recursive function with one reference " +
       "changes to the reference will change the reference", function(){
      expect(even(0)).to.be.ok();
      expect(even(1)).to.not.be.ok();
      expect(even(8)).to.be.ok();
      even = void 0;
      expect(even).to.be.eql(void 0);
    }),
    it("Given a recursive function with two reference with different name " +
       "changes to the reference will break it", function(){
      var divisableByTwo = even;
      expect(even(0)).to.be.equal(divisableByTwo(0));
      expect(even(1)).to.be.equal(divisableByTwo(1));
      expect(even(8)).to.be.equal(divisableByTwo(8));
      even = void 0;
      expect(even).to.be.eql(void 0);
      expect(divisableByTwo(0)).to.be.ok();
      expect(function(){return divisableByTwo(8);})
        .to.throwError();     // throws TypeError
      expect(function(){return divisableByTwo(0);})
        .not.to.throwError(); // to prove above works
      expect(function(){return divisableByTwo(8);})
        .to.throwException(function(e){expect(e).to.be.a(TypeError);});
      expect(function(){return divisableByTwo(8);})
        .to.throwException(/undefined is not a function/);
    });
  });
});

See the blog post that goes with this gist at
http://comp-phil.blogspot.com/2014/02/fun-with-function-references-in.html