Add proclaim.isEnumerable and proclaim.isNotEnumerable (#59)

* Add proclaim.isEnumerable and proclaim.isNotEnumerable

* Ignore lines which are only hit for really old JS engines

README.md

@@ -320,6 +320,15 @@ Assert that `actual > expected`.
 Assert that `actual >= expected`.
 
 
+### proclaim.isNotEnumerable( obj, property, [message] )
+
+Assert that `obj[property]` is not enumerable.
+
+
+### proclaim.isEnumerable( obj, property, [message] )
+
+Assert that `obj[property]` is enumerable.
+
 Why?
 ----
 

lib/proclaim.js

@@ -300,6 +300,26 @@
 		}
 	};
 
+	// Assert that a property on an object is not enumerable
+	proclaim.isNotEnumerable = function(obj, property, msg) {
+		if (arePropertyDescriptorsSupported()) {
+			if (hasWorkingGetOwnPropertyDescriptor()) {
+				proclaim.isFalse(Object.getOwnPropertyDescriptor(obj, property).enumerable, msg);
+			}
+			proclaim.isFalse(Object.prototype.propertyIsEnumerable.call(obj, property), msg);
+		}
+	};
+
+	// Assert that a property on an object is enumerable
+	proclaim.isEnumerable = function(obj, property, msg) {
+		if (arePropertyDescriptorsSupported()) {
+			if (hasWorkingGetOwnPropertyDescriptor()) {
+				proclaim.isTrue(Object.getOwnPropertyDescriptor(obj, property).enumerable, msg);
+			}
+			proclaim.isTrue(Object.prototype.propertyIsEnumerable.call(obj, property), msg);
+		}
+	};
+
 
 	// Aliases
 	// -------
@@ -359,6 +379,44 @@
 	// Utilities
 	// ---------
 
+	// Utility for checking whether property descriptors are supported
+	function arePropertyDescriptorsSupported() {
+		var obj = {};
+		try {
+			Object.defineProperty(obj, 'x', {
+				enumerable: false,
+				value: obj
+			});
+			/* eslint-disable no-unused-vars, no-restricted-syntax, guard-for-in */
+			for (var _ in obj) {
+				/* istanbul ignore next */
+				return false;
+			}
+			/* eslint-enable no-unused-vars, no-restricted-syntax, guard-for-in */
+			return true;
+		} catch (error) { // this is IE 8.
+			/* istanbul ignore next */
+			return false;
+		}
+	}
+
+	// Utility for checking whether Object.getOwnPropertyDescriptor works correctly
+	function hasWorkingGetOwnPropertyDescriptor() {
+		return 'getOwnPropertyDescriptor' in Object && typeof Object.getOwnPropertyDescriptor === 'function' && (function() {
+			try {
+				var object = {};
+				object.test = 0;
+				return Object.getOwnPropertyDescriptor(
+					object,
+					'test'
+				).value === 0;
+			} catch (exception) {
+				/* istanbul ignore next */
+				return false;
+			}
+		}());
+	}
+
 	// Utility for checking whether a value is undefined or null
 	function isUndefinedOrNull(val) {
 		return (val === null || typeof val === 'undefined');

test/unit/lib/proclaim.js

@@ -1,4 +1,5 @@
 /* eslint strict: ['error', 'function'] */
+
 (function() {
 	'use strict';
 
@@ -1114,6 +1115,62 @@
 
 		});
 
+		describe('.isEnumerable()', function() {
+
+			it('should be a function', function() {
+				assert.isFunction(proclaim.isEnumerable);
+			});
+
+			it('should not throw when called with an object whose property is enumerable', function() {
+				var property = 'a';
+				var obj = {};
+				Object.defineProperty(obj, property, {
+					enumerable: true,
+					value: 1
+				});
+				assert.doesNotThrow(callFn(proclaim.isEnumerable, obj, property));
+			});
+
+			it('should throw when called with an object whose property is not enumerable', function() {
+				var property = 'a';
+				var obj = {};
+				Object.defineProperty(obj, property, {
+					enumerable: false,
+					value: 1
+				});
+				assert.throws(callFn(proclaim.isEnumerable, obj, property));
+			});
+
+		});
+
+		describe('.isNotEnumerable()', function() {
+
+			it('should be a function', function() {
+				assert.isFunction(proclaim.isNotEnumerable);
+			});
+
+			it('should not throw when called with an object whose property is not enumerable', function() {
+				var property = 'a';
+				var obj = {};
+				Object.defineProperty(obj, property, {
+					enumerable: false,
+					value: 1
+				});
+				assert.doesNotThrow(callFn(proclaim.isNotEnumerable, obj, property));
+			});
+
+			it('should throw when called with an object whose property is enumerable', function() {
+				var property = 'a';
+				var obj = {};
+				Object.defineProperty(obj, property, {
+					enumerable: true,
+					value: 1
+				});
+				assert.throws(callFn(proclaim.isNotEnumerable, obj, property));
+			});
+
+		});
+
 	});
 
 }());