Characterization tests using Mocha and expect.js to understand how Underscore.js' constant function works

javascript_underscore_constant_characterization_spec.js

var    _ = require("underscore"),
  expect = require("expect.js");

// Characterization tests using Mocha and expect.js to understand how
// underscore.js' constant function works 
// See http://comp-phil.blogspot.com/2014/03/bird-watching-with-javascript-kestrel.html 
 
describe("Verify that we understand how underscore.js' constant works", function(){
  it("Constat will return a function", function(){
    expect(_.constant("Hello world")).to.be.a("function");
  }),
  it("Constat will return a function even when given no value", function(){
    expect(_.constant()).to.be.a("function");
  }),
  it("Constat will return a function even when given undefined", function(){
    expect(_.constant(void 0)).to.be.a("function");
  }),
  it("Constat will return a function even when given null", function(){
    expect(_.constant(null)).to.be.a("function");
  }),
  it("Given two values constant will return the first", function(){
    var alwaysFirst = _.constant("First");
    expect(alwaysFirst("Second")).to.be.equal("First");
  }),
  it("Given three values constant will return the first", function(){
    var alwaysFirst = _.constant("First");
    expect(alwaysFirst("Second", "Third")).to.be.equal("First");
  }),
  it("Given one value constant will always return that value", function(){
    var always42 = _.constant(42);
    expect(always42()).to.be.equal(42);
    expect(always42()).to.be.equal(42);
    expect(always42()).to.be.equal(42);
  }),
  it("Given a function constant will return that function", function(){
    var theTruth         = function(){return true;},
      nothingButTheTruth = _.constant(theTruth);

    expect(nothingButTheTruth()).to.be.a("function");
    expect(nothingButTheTruth(false)()).to.be.ok();
    expect(nothingButTheTruth(_.identity(42))()).to.be.equal(true);
  });
});

See my blog post which goes with this gist: http://comp-phil.blogspot.com/2014/03/bird-watching-with-javascript-kestrel.html