Add DateString type

New type that preserves string input as a string, but ensures that
the string is a valid Date.

Additionally, provides a .toDate function to provide the Date
object representation of the string.

lib/date-string.js

@@ -0,0 +1,39 @@
+// Copyright IBM Corp. 2013,2016. All Rights Reserved.
+// Node module: loopback-datasource-juggler
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+'use strict';
+
+module.exports = DateString;
+
+/**
+ * A String whose value is a valid representation of a Date.
+ * Use this type if you need to preserve the format of the value and still
+ * check if it's valid.
+ * @param {String} value
+ * @constructor
+ */
+function DateString(value) {
+  if (!(this instanceof DateString)) {
+    return new DateString(value);
+  }
+
+  if (typeof(value) !== 'string') {
+    throw new Error('Input must be a string');
+  }
+  // Preserve the reference for return via .toDate
+  this.date = new Date(value);
+  if (isNaN(this.date.getTime())) {
+    throw new Error('Invalid date');
+  }
+  this.value = value;
+};
+
+/**
+ * Return the equivalent Date object for this DateString.
+ * @returns {Date} A JavaScript Date object
+ */
+DateString.prototype.toDate = function() {
+  return this.date;
+};

lib/types.js

@@ -40,6 +40,7 @@ Types.Any.prototype.toObject = Types.Any.prototype.toJSON = function() {
 };
 
 module.exports = function(modelTypes) {
+  var DateString = require('./date-string');
   var GeoPoint = require('./geo').GeoPoint;
 
   for (var t in Types) {
@@ -63,6 +64,7 @@ module.exports = function(modelTypes) {
   modelTypes.registerType(Number);
   modelTypes.registerType(Boolean);
   modelTypes.registerType(Date);
+  modelTypes.registerType(DateString);
   modelTypes.registerType(Buffer, ['Binary']);
   modelTypes.registerType(Array);
   modelTypes.registerType(GeoPoint);

test/date-string.test.js

@@ -0,0 +1,37 @@
+// Copyright IBM Corp. 2014,2016. All Rights Reserved.
+// Node module: loopback-datasource-juggler
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+/* global describe,it */
+/* jshint expr:true */
+
+'use strict';
+
+require('should');
+
+var DateString = require('../lib/date-string');
+
+describe('DateString', function() {
+  describe('constructor', function() {
+    it('should support a valid date string', function() {
+      var date = new DateString('2015-01-01');
+      date.should.not.eql(null);
+      date.value.should.eql('2015-01-01');
+      var d = new Date('2015-01-01');
+      date.toDate().toString().should.eql(d.toString());
+    });
+
+    it('should throw on invalid input', function() {
+      var fn = () => {
+        var date = new DateString('notadate');
+      };
+      fn.should.throw(new Error('Invalid date'));
+
+      fn = () => {
+        var date = new DateString(20150101);
+      };
+      fn.should.throw(new Error('Input must be a string'));
+    });
+  });
+});