sebastienros · lahma · Aug 1, 2026 · Aug 1, 2026
diff --git a/Jint/Native/Date/DatePrototype.cs b/Jint/Native/Date/DatePrototype.cs
@@ -968,7 +968,7 @@ private JsValue ToJson(JsValue thisObject, JsCallArguments arguments)
     {
         var o = TypeConverter.ToObject(_realm, thisObject);
         var tv = TypeConverter.ToPrimitive(o, Types.Number);
-        if (tv.IsNumber() && !IsFinite(((JsNumber) tv)._value))
+        if (tv.IsNumber() && !double.IsFinite(((JsNumber) tv)._value))
         {
             return Null;
         }
@@ -1420,19 +1420,16 @@ internal static DatePresentation MakeDate(double day, double time)
         return new DatePresentation((long) (day * MsPerDay + time), DateFlags.None);
     }
 
-    [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    private static bool IsFinite(double value) => !double.IsNaN(value) && !double.IsInfinity(value);
-
-    private static bool AreFinite(double value1, double value2) => IsFinite(value1) && IsFinite(value2);
+    private static bool AreFinite(double value1, double value2) => double.IsFinite(value1) && double.IsFinite(value2);
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static bool IsFinite(DatePresentation value) => value.IsFinite;
 
     private static bool AreFinite(double value1, double value2, double value3)
-        => IsFinite(value1) && IsFinite(value2) && IsFinite(value3);
+        => double.IsFinite(value1) && double.IsFinite(value2) && double.IsFinite(value3);
 
     private static bool AreFinite(double value1, double value2, double value3, double value4)
-        => IsFinite(value1) && IsFinite(value2) && IsFinite(value3) && IsFinite(value4);
+        => double.IsFinite(value1) && double.IsFinite(value2) && double.IsFinite(value3) && double.IsFinite(value4);
 
     [StructLayout(LayoutKind.Auto)]
     private readonly record struct Date(int Year, int Month, int Day);

diff --git a/Jint/Native/Global/GlobalObject.cs b/Jint/Native/Global/GlobalObject.cs
@@ -262,7 +262,7 @@ private static JsValue IsNaN(JsValue thisObject, JsValue value)
     private static JsValue IsFinite(JsValue thisObject, JsValue value)
     {
         var n = TypeConverter.ToNumber(value);
-        return !double.IsNaN(n) && !double.IsInfinity(n);
+        return double.IsFinite(n);
     }
 
     private const string UriReservedString = ";/?:@&=+$,";

diff --git a/Jint/Native/Intl/DurationFormatPrototype.cs b/Jint/Native/Intl/DurationFormatPrototype.cs
@@ -281,7 +281,7 @@ private double GetDurationComponent(ObjectInstance obj, string property)
         }
 
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, $"Invalid value for {property}");
         }

diff --git a/Jint/Native/Intl/JsNumberFormat.cs b/Jint/Native/Intl/JsNumberFormat.cs
@@ -208,7 +208,7 @@ private string FormatWithNotation(double value)
 
     private string FormatScientific(double value)
     {
-        if (double.IsNaN(value) || double.IsInfinity(value) || value == 0)
+        if (!double.IsFinite(value) || value == 0)
         {
             return FormatDecimal(value);
         }
@@ -232,7 +232,7 @@ private string FormatScientific(double value)
 
     private string FormatEngineering(double value)
     {
-        if (double.IsNaN(value) || double.IsInfinity(value) || value == 0)
+        if (!double.IsFinite(value) || value == 0)
         {
             return FormatDecimal(value);
         }
@@ -702,7 +702,7 @@ private void AddDecimalParts(List<NumberFormatPart> parts, double value)
     /// </summary>
     private double ApplyRounding(double value, int decimalPlaces)
     {
-        if (double.IsNaN(value) || double.IsInfinity(value))
+        if (!double.IsFinite(value))
         {
             return value;
         }
@@ -1214,7 +1214,7 @@ private string FormatDecimal(double value)
 
     private string FormatWithSignificantDigits(double value)
     {
-        if (double.IsNaN(value) || double.IsInfinity(value))
+        if (!double.IsFinite(value))
         {
             return value.ToString(NumberFormatInfo);
         }

diff --git a/Jint/Native/Intl/JsPluralRules.cs b/Jint/Native/Intl/JsPluralRules.cs
@@ -135,7 +135,7 @@ internal string Select(double n)
     private string SelectCardinal(double n)
     {
         // Handle special values
-        if (double.IsNaN(n) || double.IsInfinity(n))
+        if (!double.IsFinite(n))
         {
             return "other";
         }
@@ -194,7 +194,7 @@ private string SelectCardinal(double n)
     /// </summary>
     private string SelectOrdinal(double n)
     {
-        if (double.IsNaN(n) || double.IsInfinity(n))
+        if (!double.IsFinite(n))
         {
             return "other";
         }

diff --git a/Jint/Native/Intl/NumberFormatConstructor.cs b/Jint/Native/Intl/NumberFormatConstructor.cs
@@ -668,7 +668,7 @@ private int GetRoundingIncrementOption(ObjectInstance options)
         }
 
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, "roundingIncrement must be a finite number");
         }

diff --git a/Jint/Native/Intl/RelativeTimeFormatPrototype.cs b/Jint/Native/Intl/RelativeTimeFormatPrototype.cs
@@ -53,7 +53,7 @@ private JsValue Format(JsValue thisObject, JsValue value, JsValue unit)
         var relativeTimeFormat = ValidateRelativeTimeFormat(thisObject);
 
         var numericValue = TypeConverter.ToNumber(value);
-        if (double.IsNaN(numericValue) || double.IsInfinity(numericValue))
+        if (!double.IsFinite(numericValue))
         {
             Throw.RangeError(_realm, "Invalid value");
         }
@@ -77,7 +77,7 @@ private JsArray FormatToParts(JsValue thisObject, JsValue value, JsValue unit)
         var relativeTimeFormat = ValidateRelativeTimeFormat(thisObject);
 
         var numericValue = TypeConverter.ToNumber(value);
-        if (double.IsNaN(numericValue) || double.IsInfinity(numericValue))
+        if (!double.IsFinite(numericValue))
         {
             Throw.RangeError(_realm, "Invalid value");
         }

diff --git a/Jint/Native/Json/JsonSerializer.cs b/Jint/Native/Json/JsonSerializer.cs
@@ -401,7 +401,7 @@ private SerializeResult SerializeJSONValue(JsValue value, ref ValueStringBuilder
                 return SerializeResult.NotUndefined;
             }
 
-            var isFinite = !double.IsNaN(doubleValue) && !double.IsInfinity(doubleValue);
+            var isFinite = double.IsFinite(doubleValue);
             if (isFinite)
             {
                 if (TypeConverter.CanBeStringifiedAsLong(doubleValue))

diff --git a/Jint/Native/Temporal/Duration/DurationPrototype.cs b/Jint/Native/Temporal/Duration/DurationPrototype.cs
@@ -122,7 +122,7 @@ private double GetOptionalDurationProperty(ObjectInstance obj, string name, doub
         anyDefined = true;
 
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, $"Duration {name} must be a finite number");
         }
@@ -278,7 +278,7 @@ private JsDuration Round(JsValue thisObject, JsValue options)
             if (!roundingIncrementValue.IsUndefined())
             {
                 roundingIncrement = TypeConverter.ToNumber(roundingIncrementValue);
-                if (double.IsNaN(roundingIncrement) || double.IsInfinity(roundingIncrement))
+                if (!double.IsFinite(roundingIncrement))
                 {
                     Throw.RangeError(_realm, "roundingIncrement must be a finite number");
                 }
@@ -1381,7 +1381,7 @@ private JsString ToStringMethod(JsValue thisObject, JsValue options)
                 if (fsdValue.IsNumber())
                 {
                     var fsdNum = fsdValue.AsNumber();
-                    if (double.IsNaN(fsdNum) || double.IsInfinity(fsdNum))
+                    if (!double.IsFinite(fsdNum))
                     {
                         Throw.RangeError(_realm, "fractionalSecondDigits must be 'auto' or a number 0-9");
                     }

diff --git a/Jint/Native/Temporal/Instant/InstantConstructor.cs b/Jint/Native/Temporal/Instant/InstantConstructor.cs
@@ -55,7 +55,7 @@ private JsInstant FromEpochMilliseconds(JsValue thisObject, JsValue epochMillise
         var ms = TypeConverter.ToNumber(epochMilliseconds);
 
         // NumberToBigInt: must be an integral number
-        if (double.IsNaN(ms) || double.IsInfinity(ms))
+        if (!double.IsFinite(ms))
         {
             Throw.RangeError(_realm, "Invalid epoch milliseconds");
         }

diff --git a/Jint/Native/Temporal/Instant/InstantPrototype.cs b/Jint/Native/Temporal/Instant/InstantPrototype.cs
@@ -190,7 +190,7 @@ private JsDuration DifferenceTemporalInstant(JsValue thisObject, JsCallArguments
             if (!incValue.IsUndefined())
             {
                 var inc = TypeConverter.ToNumber(incValue);
-                if (double.IsNaN(inc) || double.IsInfinity(inc))
+                if (!double.IsFinite(inc))
                 {
                     Throw.RangeError(_realm, "roundingIncrement must be finite");
                 }
@@ -323,7 +323,7 @@ private JsInstant Round(JsValue thisObject, JsValue roundTo)
         if (!roundingIncrementValue.IsUndefined())
         {
             var inc = TypeConverter.ToNumber(roundingIncrementValue);
-            if (double.IsNaN(inc) || double.IsInfinity(inc))
+            if (!double.IsFinite(inc))
             {
                 Throw.RangeError(_realm, "roundingIncrement must be finite");
             }
@@ -561,7 +561,7 @@ private JsString ToStringMethod(JsValue thisObject, JsValue options)
                 else
                 {
                     var num = ((JsNumber) digitsValue)._value;
-                    if (double.IsNaN(num) || double.IsInfinity(num))
+                    if (!double.IsFinite(num))
                     {
                         Throw.RangeError(_realm, "fractionalSecondDigits must be finite");
                     }

diff --git a/Jint/Native/Temporal/PlainDateTime/PlainDateTimePrototype.cs b/Jint/Native/Temporal/PlainDateTime/PlainDateTimePrototype.cs
@@ -799,7 +799,7 @@ private JsString ToString(JsValue thisObject, JsValue options)
                 if (fsdValue.IsNumber())
                 {
                     var fsdNum = fsdValue.AsNumber();
-                    if (double.IsNaN(fsdNum) || double.IsInfinity(fsdNum))
+                    if (!double.IsFinite(fsdNum))
                     {
                         Throw.RangeError(_realm, "fractionalSecondDigits must be a finite number");
                     }
@@ -1141,7 +1141,7 @@ private static string FormatDateTime(IsoDateTime dateTime, string calendar, stri
     private int ConvertToInteger(JsValue value, string fieldName)
     {
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, $"DateTime {fieldName} must be a finite number");
         }
@@ -1239,7 +1239,7 @@ private double GetDurationProperty(ObjectInstance obj, string name, ref bool has
         hasAny = true;
 
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, $"Duration {name} must be a finite number");
         }

diff --git a/Jint/Native/Temporal/PlainTime/PlainTimeConstructor.cs b/Jint/Native/Temporal/PlainTime/PlainTimeConstructor.cs
@@ -257,7 +257,7 @@ private int GetTimeProperty(ObjectInstance obj, string name, int defaultValue, r
         hasAny = true;
 
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, $"Time {name} must be a finite number");
         }

diff --git a/Jint/Native/Temporal/PlainTime/PlainTimePrototype.cs b/Jint/Native/Temporal/PlainTime/PlainTimePrototype.cs
@@ -409,7 +409,7 @@ private JsString ToString(JsValue thisObject, JsValue options)
                 if (fsdValue.IsNumber())
                 {
                     var fsdNum = fsdValue.AsNumber();
-                    if (double.IsNaN(fsdNum) || double.IsInfinity(fsdNum))
+                    if (!double.IsFinite(fsdNum))
                     {
                         Throw.RangeError(_realm, "fractionalSecondDigits must be a finite number");
                     }

diff --git a/Jint/Native/Temporal/TemporalHelpers.cs b/Jint/Native/Temporal/TemporalHelpers.cs
@@ -1134,7 +1134,7 @@ public static bool IsValidDuration(DurationRecord duration)
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static bool CheckDurationComponent(double component, ref int sign)
     {
-        if (double.IsNaN(component) || double.IsInfinity(component))
+        if (!double.IsFinite(component))
             return false;
 
         if (component > 0)
@@ -4944,7 +4944,7 @@ public static int GetRoundingIncrementOption(Realm realm, JsValue normalizedOpti
         var number = TypeConverter.ToNumber(increment);
 
         // 6. If integerIncrement is NaN, +∞, or -∞, throw a RangeError exception.
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(realm, "Rounding increment must be finite");
         }

diff --git a/Jint/Native/Temporal/ZonedDateTime/ZonedDateTimeConstructor.cs b/Jint/Native/Temporal/ZonedDateTime/ZonedDateTimeConstructor.cs
@@ -982,7 +982,7 @@ private BigInteger ToBigInt(JsValue value)
         }
 
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, "epochNanoseconds must be a finite number");
         }

diff --git a/Jint/Native/Temporal/ZonedDateTime/ZonedDateTimePrototype.cs b/Jint/Native/Temporal/ZonedDateTime/ZonedDateTimePrototype.cs
@@ -965,7 +965,7 @@ private JsString ToStringMethod(JsValue thisObject, JsValue options)
                 else
                 {
                     var num = ((JsNumber) precisionProp)._value;
-                    if (double.IsNaN(num) || double.IsInfinity(num))
+                    if (!double.IsFinite(num))
                     {
                         Throw.RangeError(_realm, "fractionalSecondDigits must be finite");
                     }
@@ -1398,7 +1398,7 @@ private double GetDoubleProperty(ObjectInstance obj, string name, double default
         hasAny = true;
 
         var number = TypeConverter.ToNumber(value);
-        if (double.IsNaN(number) || double.IsInfinity(number))
+        if (!double.IsFinite(number))
         {
             Throw.RangeError(_realm, $"Duration {name} must be a finite number");
         }

diff --git a/Jint/Runtime/Interpreter/Expressions/JintExpression.cs b/Jint/Runtime/Interpreter/Expressions/JintExpression.cs
@@ -227,7 +227,9 @@ protected static JsValue Remainder(EvaluationContext context, JsValue left, JsVa
                 var n = left.AsNumber();
                 var d = right.AsNumber();
 
-                if (double.IsNaN(n) || double.IsNaN(d) || double.IsInfinity(n))
+                // A non-finite dividend, or a NaN divisor, gives NaN. An infinite *divisor* does not,
+                // and is handled by the branch below, so only n gets the full finiteness test.
+                if (!double.IsFinite(n) || double.IsNaN(d))
                 {
                     result = JsNumber.DoubleNaN;
                 }

diff --git a/Jint/Runtime/TypeConverter.cs b/Jint/Runtime/TypeConverter.cs
@@ -1022,12 +1022,12 @@ public static ObjectInstance ToObject(Realm realm, JsValue value)
     internal static bool IsIntegralNumber(double value)
     {
         // Math.Floor(value) == value instead of value % 1 == 0: the remainder operator on doubles
-        // compiles to a native fmod call, Math.Floor is a JIT intrinsic (a single vroundsd). The NaN
-        // and infinity guards above still short-circuit, and for every finite value the two tests are
+        // compiles to a native fmod call, Math.Floor is a JIT intrinsic (a single vroundsd). The
+        // finiteness guard still short-circuits, and for every finite value the two tests are
         // equivalent - value % 1 is value - trunc(value), which is +-0 exactly when value has no
         // fractional part, which is exactly when Math.Floor(value) == value (-0.0 and magnitudes
         // >= 2^52, which are necessarily integral, are accepted by both).
-        return !double.IsNaN(value) && !double.IsInfinity(value) && Math.Floor(value) == value;
+        return double.IsFinite(value) && Math.Floor(value) == value;
     }
 
     private static ObjectInstance ToObjectNonObject(Realm realm, JsValue value)