Implement Date.setMonth/setUTCMonth

global.go

@@ -499,8 +499,54 @@ func newContext() *_runtime {
 		// setUTCHours
 		// setDate
 		// setUTCDate
-		// setMonth
+		"setMonth", 1, func(call FunctionCall) Value {
-		// setUTCMonth
+			date := dateObjectOf(call.thisObject())
+			if date.isNaN {
+				return NaNValue()
+			}
+			value := call.Argument(0)
+			if value.IsNaN() {
+				date.SetNaN()
+				return NaNValue()
+			}
+			baseTime := date.Time().Local()
+			setTime := time_.Date(
+				baseTime.Year(),
+				dateToGoMonth(int(toInteger(value))),
+				baseTime.Day(),
+				baseTime.Hour(),
+				baseTime.Minute(),
+				baseTime.Second(),
+				baseTime.Nanosecond(),
+				baseTime.Location(),
+			)
+			date.SetTime(setTime)
+			return date.Value()
+		},
+		"setUTCMonth", 1, func(call FunctionCall) Value {
+			date := dateObjectOf(call.thisObject())
+			if date.isNaN {
+				return NaNValue()
+			}
+			value := call.Argument(0)
+			if value.IsNaN() {
+				date.SetNaN()
+				return NaNValue()
+			}
+			baseTime := date.Time().UTC()
+			setTime := time_.Date(
+				baseTime.Year(),
+				dateToGoMonth(int(toInteger(value))),
+				baseTime.Day(),
+				baseTime.Hour(),
+				baseTime.Minute(),
+				baseTime.Second(),
+				baseTime.Nanosecond(),
+				baseTime.Location(),
+			)
+			date.SetTime(setTime)
+			return date.Value()
+		},
 		"setFullYear", 1, func(call FunctionCall) Value {
 			date := dateObjectOf(call.thisObject())
 			if date.isNaN {

otto_test.go

@@ -578,6 +578,10 @@ func TestDate(t *testing.T) {
 	test(`new Date(12564504e5).toString()`, "Sun, 25 Oct 2009 06:00:00 UTC")
 	test(`new Date(2009, 9, 25).toString()`, "Sun, 25 Oct 2009 00:00:00 UTC")
 	test(`+(new Date(2009, 9, 25))`, "1.2564288e+12")
+
+	test(`abc = new Date(12564504e5); abc.setMonth(9); abc.toString()`, "Sun, 25 Oct 2009 06:00:00 UTC")
+	test(`abc = new Date(12564504e5); abc.setMonth("09"); abc.toString()`, "Sun, 25 Oct 2009 06:00:00 UTC")
+	test(`abc = new Date(12564504e5); abc.setMonth("10"); abc.toString()`, "Wed, 25 Nov 2009 07:00:00 UTC")
 }
 
 func TestComparison(t *testing.T) {

type_date.go

@@ -88,7 +88,7 @@ func dateObjectOf(_dateObject *_object) *_dateObject {
 	return _dateObject.Date
 }
 
-// JavaScript is 0-based, Go is 1-based
+// JavaScript is 0-based, Go is 1-based (15.9.1.4)
 func dateToGoMonth(month int) tme.Month {
 	return tme.Month(month + 1)
 }