add tests for .to() with relative array values, and implement it

Author: trusktr

src/Tween.js

@@ -204,6 +204,19 @@ TWEEN.Tween.prototype = {
 					continue;
 				}
 
+				// handle relative values
+				this._valuesEnd[property] = this._valuesEnd[property].map(n => {
+					if (typeof n !== 'string') {
+						return n;
+					}
+
+					if (n.charAt(0) === '+' || n.charAt(0) === '-') {
+						return this._object[property] + parseFloat(n);
+					} else {
+						return parseFloat(n);
+					}
+				});
+
 				// Create a local copy of the Array with the start value at the front
 
 				this._valuesEnd[property] = [this._object[property]].concat(this._valuesEnd[property]);

test/unit/tests.js

@@ -433,6 +433,49 @@
 
 			},
 
+			'Tween relative positive array values': function(test) {
+
+				var obj = { x: 0 },
+					t = new TWEEN.Tween( obj );
+
+				t.to( { x: ["+100", "+0", "-100", "+0"] }, 2000 );
+				t.start( 0 );
+
+				t.update( 250 );
+				test.equal( obj.x, 50 );
+				t.update( 500 );
+				test.equal( obj.x, 100 );
+				t.update( 750 );
+				test.equal( obj.x, 50 );
+				t.update( 1000 );
+				test.equal( obj.x, 0 );
+				t.update( 1250 );
+				test.equal( obj.x, -50 );
+				t.update( 1500 );
+				test.equal( obj.x, -100 );
+				t.update( 1750 );
+				test.equal( obj.x, -50 );
+				t.update( 2000 );
+				test.equal( obj.x, 0 );
+
+				test.done();
+
+			},
+
+			// 'String values without a + or - sign should not be interpreted as relative with array values': function(test) {
+
+			// 	var obj = { x: 100 },
+			// 		t = new TWEEN.Tween( obj );
+
+			// 	t.to( { x: "100" }, 1000 );
+			// 	t.start( 0 );
+			// 	t.update( 1000 );
+
+			// 	test.equal( obj.x, 100 );
+			// 	test.done();
+
+			// },
+
 			'Test TWEEN.Tween.start()': function(test) {
 
 				var obj = { },