Allow empty test/update in for statements

Fix #1

evaluate_statement.go

@@ -150,17 +150,21 @@ func (self *_runtime) evaluateFor(node *_forNode) Value {
 	return self.breakEvaluate(_labelSet, func() Value {
 		result := emptyValue()
 		for {
-			testResult := self.evaluate(test)
-			testResultValue := self.GetValue(testResult)
-			if (toBoolean(testResultValue) == false) {
-				break
+			if test != nil {
+				testResult := self.evaluate(test)
+				testResultValue := self.GetValue(testResult)
+				if (toBoolean(testResultValue) == false) {
+					break
+				}
 			}
 			value, _ := self.continueEvaluate(body, _labelSet)
 			if !value.isEmpty() {
 				result = value
 			}
-			updateResult := self.evaluate(update)
-			self.GetValue(updateResult) // Side-effect trigger
+			if update != nil {
+				updateResult := self.evaluate(update)
+				self.GetValue(updateResult) // Side-effect trigger
+			}
 		}
 		return result
 	})

otto_test.go

@@ -773,6 +773,34 @@ func TestFor(t *testing.T) {
 		}
 	`)
 	test("result", "9")
+
+	test(`
+		abc = 0
+		for (;;) {
+			abc += 1
+			if (abc == 3)
+				break
+		}
+		abc
+	`, "3")
+
+	test(`
+		for (abc = 0; ;) {
+			abc += 1
+			if (abc == 3)
+				break
+		}
+		abc
+	`, "3")
+
+	test(`
+		for (abc = 0; ; abc+=1) {
+			abc += 1
+			if (abc == 3)
+				break
+		}
+		abc
+	`, "3")
 }
 
 func TestArguments(t *testing.T) {
@@ -1844,3 +1872,4 @@ func TestAPI(t *testing.T) {
 	result, _ = object.Value().Call(def, 3)
 	Is(result, "30")
 }
+

parser_test.go

@@ -637,7 +637,6 @@ func TestParseSuccess(t *testing.T) {
 	---
 	{ @ xyzzy { <throw> { <new> TypeError "Nothing happens." } } }
 	`)
-
 }
 
 func TestParseFailure(t *testing.T) {