Support diffs

lib/assertThat.js

@@ -1,6 +1,7 @@
 'use strict';
 
 var q = require('q');
+var _ = require('lodash');
 var AssertionError = require('assertion-error')
 	, Description = require('./Description')
 	;
@@ -25,7 +26,18 @@ function assertThat(reason, actual, matcher) {
 			.appendDescriptionOf(matcher)
 			.append('\n     but: ');
 		matcher.describeMismatch(actual, description);
-		throw new AssertionError(description.get(), {}, assertThat);
+
+		var errorProperties = {};
+		if (_.isFunction(matcher.getExpectedForDiff) &&
+			_.isFunction(matcher.formatActualForDiff)) {
+			errorProperties = {
+				showDiff: true,
+				expected: matcher.getExpectedForDiff(),
+				actual: matcher.formatActualForDiff(actual)
+			};
+		}
+
+		throw new AssertionError(description.get(), errorProperties, assertThat);
 	}
 }
 

lib/matchers/Is.js

@@ -17,7 +17,9 @@ var Is = acceptingMatcher(function Is(innerMatcher) {
 		},
 		describeMismatch: function (value, description) {
 			return innerMatcher.describeMismatch(value, description);
-		}
+		},
+		getExpectedForDiff: innerMatcher.getExpectedForDiff,
+		formatActualForDiff: innerMatcher.formatActualForDiff
 	});
 });
 

lib/matchers/IsEqual.js

@@ -11,7 +11,9 @@ function IsEqual(expectedValue) {
 		},
 		describeTo: function (description) {
 			description.appendValue(expectedValue);
-		}
+		},
+		getExpectedForDiff: function () { return expectedValue; },
+		formatActualForDiff: function (actual) { return actual; }
 	});
 }
 

lib/matchers/IsObjectWithProperties.js

@@ -56,6 +56,12 @@ function IsObjectWithProperties(properties) {
 					.append(' ');
 				return propertyMatchers[key].describeMismatch(actual[key], description);
 			});
+		},
+		getExpectedForDiff: function () {
+			return properties;
+		},
+		formatActualForDiff: function (actual) {
+			return actual;
 		}
 	});
 }

lib/matchers/SubstringMatcher.js

@@ -25,7 +25,9 @@ function SubstringMatcher(substring, relation, matchesString) {
 			description
 				.append('was ')
 				.appendValue(actual);
-		}
+		},
+		getExpectedForDiff: function () { return substring; },
+		formatActualForDiff: function (actual) { return actual; }
 	});
 }
 

lib/promiseThat.js

@@ -3,6 +3,7 @@
 var q = require('q');
 var AssertionError = require('assertion-error');
 var Description = require('./Description');
+var _ = require('lodash');
 
 function promiseThat(reason, actual, matcher) {
 	if (arguments.length === 2) {
@@ -19,7 +20,23 @@ function promiseThat(reason, actual, matcher) {
 				.appendDescriptionOf(matcher)
 				.append('\n     but: ');
 			return q(matcher.describeMismatch(actual, description)).then(function () {
-				throw new AssertionError(description.get(), {}, promiseThat);
+				if (!_.isFunction(matcher.getExpectedForDiff) ||
+					!_.isFunction(matcher.formatActualForDiff)) {
+					return {};
+				}
+
+				return q.all([
+					matcher.getExpectedForDiff(),
+					matcher.formatActualForDiff(actual)
+				]).spread(function (expected, actual) {
+					return {
+						showDiff: true,
+						expected: expected,
+						actual: actual
+					};
+				});
+			}).then(function (errorProperties) {
+				throw new AssertionError(description.get(), errorProperties, promiseThat);
 			});
 		}
 	});

package.json

@@ -41,10 +41,10 @@
     "grunt-contrib-nodeunit": "^0.2.2",
     "grunt-contrib-uglify": "^0.3.3",
     "grunt-contrib-watch": "^0.5.3",
-    "grunt-mocha-test": "^0.12.1",
+    "grunt-mocha-test": "^0.12.4",
     "jshint-stylish": "^0.1.5",
     "load-grunt-tasks": "^0.2.1",
-    "mocha": "^1.21.4",
+    "mocha": "^2.0.1",
     "time-grunt": "^0.2.10"
   },
   "dependencies": {

test/assertThatSpec.js

@@ -59,6 +59,28 @@ describe('assertThat', function () {
 		assertEquals(thrown.message , 'Assertion message\nExpected: Matcher description\n     but: was "real value"');
 	});
 
+	it('should pass diff representations to AssertionError', function () {
+		var thrown;
+
+		var testMatcher = new TestMatcher(function () { return false; });
+		testMatcher.getExpectedForDiff = function () {
+			return 'expected for diff';
+		};
+		testMatcher.formatActualForDiff = function (actual) {
+			return 'actual for diff: ' + actual;
+		};
+
+		try {
+			assertThat('actual value', testMatcher);
+		}
+		catch (e) {
+			thrown = e;
+		}
+
+		assertEquals(thrown.expected, 'expected for diff');
+		assertEquals(thrown.actual, 'actual for diff: actual value');
+	});
+
 	it('should throw if matcher returns a promise', function () {
 		var thrown;
 

test/matchers/IsObjectWithPropertiesSpec.js

@@ -80,6 +80,16 @@ describe('IsObjectWithProperties', function () {
 			});
 		});
 
+		it('should format actual for diff', function () {
+			var simple = IsObjectWithProperties.hasProperties();
+			__.assertThat(simple.formatActualForDiff({a: 1}), __.equalTo({a: 1}));
+		});
+
+		it('should provide expected for diff', function () {
+			var simple = IsObjectWithProperties.hasProperties({a: 1});
+			__.assertThat(simple.getExpectedForDiff(), __.equalTo({a: 1}));
+		});
+
 		describe('with a promising matcher', function () {
 			beforeEach(function () {
 				sut = hasProperties({

test/matchers/SubstringMatcherSpec.js

@@ -6,6 +6,7 @@ var AssertionError = require('assertion-error')
 	, __ = require('../../lib/hamjest')
 	, assertTrue = require('../asserts').assertTrue
 	, assertFalse = require('../asserts').assertFalse
+	, assertEquals = require('../asserts').assertEquals
 	;
 
 describe('SubstringMatcher', function () {
@@ -40,6 +41,14 @@ describe('SubstringMatcher', function () {
 			assertFalse(sut.matches(5));
 		});
 
+		it('should provide expected for diff', function () {
+			assertEquals('a value', sut.getExpectedForDiff());
+		});
+
+		it('should format actual for diff', function() {
+			assertEquals('foo', sut.formatActualForDiff('foo'));
+		});
+
 		describe('description', function () {
 			var description;
 

test/promiseThatSpec.js

@@ -157,4 +157,27 @@ describe('promiseThat', function () {
 
 		deferred.resolve();
 	});
+
+	it('should pass diff representations to AssertionError', function (done) {
+		var testMatcher = new TestMatcher(function () { return false; });
+		testMatcher.getExpectedForDiff = function () {
+			return 'expected for diff';
+		};
+		testMatcher.formatActualForDiff = function (actual) {
+			return q('actual for diff: ' + actual);
+		};
+
+		promiseThat('actual value', testMatcher).done(
+			function () {
+				fail('Should not be fulfilled');
+			},
+			function (reason) {
+				__.assertThat(reason, __.hasProperties({
+					expected: 'expected for diff',
+					actual: 'actual for diff: actual value'
+				}));
+				done();
+			}
+		);
+	});
 });