Characterization tests using Mocha and expect.js to understand how underscore compose works.

javascript_bluebird_characterization_spec.js

var expect = require("expect.js"),
         _ = require("underscore"),
 increment = function(x){return ++x;};
 
// Characterization tests using Mocha and expect.js to understand how
// underscore compose works
// See http://comp-phil.blogspot.com/2014/02/bird-watching-with-javascript-bluebird.html

describe("Let's figure out how underscore's compose function works", function(){
  describe("Simple function examples", function(){
    it("Given a bunch of identity functions it will return the value given", function(){
      var ids = _.compose(_.identity, _.identity, _.identity, _.identity);
      expect(ids("Mike Harris")).to.be.equal("Mike Harris");
    }),
    it("Given an increment function it will return one more", function(){
      var plus1 = _.compose(increment);
      expect(plus1(42)).to.be.equal(43);
    }),
    it("Given an increment and identity function it will return one more", function(){
      var plus1 = _.compose(_.identity, increment);
      expect(plus1(42)).to.be.equal(43);
    });
  }),
  describe("Complex functions", function(){
    it("Given a absolute value and square root functions we can find positive square roots", function(){
        var posSqrt = _.compose(Math.sqrt, Math.abs);
        expect(posSqrt(-4)).to.equal(2);
    }),
    it("Given a absolute value and square root functions but used in the wrong order " +
       "we cannot find positive square roots", function(){
        var wrongPosSqrt = _.compose(Math.abs, Math.sqrt);
        expect(_.isNaN(wrongPosSqrt(-4))).to.be.ok();

        // even better
        var failedPosSqrt = _.compose(_.isNaN, wrongPosSqrt);
        expect(failedPosSqrt(-4)).to.be.ok();

        // or
        var showWhenFailed = _.compose(function(x){return !x;}, failedPosSqrt);
        expect(showWhenFailed(2)).to.be.ok();
        expect(showWhenFailed(-2)).to.not.be.ok();
    }),
    it("Given a greet and exclaim function we can really greet someone!", function(){
      // based on example from http://underscorejs.org/#compose
      var greet = function(name){ return "Hi " + name; },
        exclaim = function(statement){ return statement + "!"; },
        welcome = _.compose(greet, exclaim);
        expect(welcome("Mr. Jack")).to.equal("Hi Mr. Jack!");
    });
  });
});

See the blog post that goes with this gist at
http://comp-phil.blogspot.com/2014/02/bird-watching-with-javascript-bluebird.html