PropertyGetter monad kata

property-getter.js

function PropertyGetterFactory(value) {
  return {
    get: function(prop) {
      return mapToPropertyGetter(partial(get, prop), value);
    },
    map: function(callback) {
      return mapToPropertyGetter(callback, value);
    },
    unwrap: always(value)
  };
}

function mapToPropertyGetter(callback, value) {
  return PropertyGetterFactory(map(callback, value));
}

function get(prop, obj) {
  return obj[prop];
}

function partial(fn, value) {
  return fn.bind(null, value);
}

function map(callback, value) {
  if(isNil(value)) {
    return value;
  } else {
    return callback(value);
  }
}

function always(v) {
  return function() {
    return v;
  };
}

function isNil(v) {
  return v === null || v === undefined;
}

module.exports = PropertyGetterFactory;

property-getter_spec.js

var R = require('ramda');

var PropertyGetterFactory = require('./property-getter');

ddescribe('PropertyGetter', function() {
  var data = {
    title: 'This is a title'
  };

  var PropertyGetter = PropertyGetterFactory(data);

  function toBeAPropertyGetterWithValue(expected) {
    return R.eqDeep(this.actual.unwrap(), expected);
  }

  beforeEach(function() {
    this.addMatchers({
      toBeAPropertyGetterWithValue: toBeAPropertyGetterWithValue
    });
  });

  it('should return a property on an object', function() {
    expect(PropertyGetter.get('title')).toBeAPropertyGetterWithValue(data.title);
  });

  it('should return undefined if the initial value is undefined', function() {
    PropertyGetter = PropertyGetterFactory(undefined);
    expect(PropertyGetter.get('title')).toBeAPropertyGetterWithValue(undefined);
  });

  it('should return undefined if the initial value does not have the defined property', function() {
    PropertyGetter = PropertyGetterFactory(5);
    expect(PropertyGetter.get('title')).toBeAPropertyGetterWithValue(undefined);
  });

  it('should return an array item if the initial value is an array', function() {
    PropertyGetter = PropertyGetterFactory([1, 2]);
    expect(PropertyGetter.get('0')).toBeAPropertyGetterWithValue(1);
  });
});