Add isInteger to jsi::Value (facebook#56161)

Summary:
Pull Request resolved: facebook#56161

Add a `isInteger` method under `jsi::Value` that returns true iff
`Number.isInteger` returns true.

This can be implemented without relying on the Runtime.

Changelog: [Internal]

Reviewed By: lavenzg

Differential Revision: D96175707

Author: tsaichien

packages/react-native/ReactCommon/jsi/jsi/jsi.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <cassert>
+#include <cmath>
 #include <cstdint>
 #include <cstring>
 #include <exception>
@@ -1899,6 +1900,28 @@ class JSI_EXPORT Value {
     return kind_ == ObjectKind;
   }
 
+  /// \returns true if `Number.isInteger(value)` returns true, false otherwise.
+  bool isInteger() const {
+    // 1. If number is an integral Number, return true.
+    // 2. Return false.
+
+    // The spec's definition of integral number:
+    // When the term integer is used in this specification, it refers to a
+    // mathematical value which is in the set of integers, unless otherwise
+    // stated. When the term integral Number is used in this specification, it
+    // refers to a finite Number value whose mathematical value is in the set of
+    // integers.
+
+    if (!isNumber()) {
+      return false;
+    }
+    auto number = data_.number;
+    if (!std::isfinite(number)) {
+      return false;
+    }
+    return number == std::trunc(number);
+  }
+
   /// \return the boolean value, or asserts if not a boolean.
   bool getBool() const {
     assert(isBool());

packages/react-native/ReactCommon/jsi/jsi/test/testlib.cpp

@@ -974,6 +974,42 @@ TEST_P(JSITest, ValueTest) {
   EXPECT_EQ(eval("['zero',1,2,3]").toString(rt).utf8(rt), "zero,1,2,3");
 }
 
+TEST_P(JSITest, IsIntegerTest) {
+  // Non-number values should return false
+  EXPECT_FALSE(Value::undefined().isInteger());
+  EXPECT_FALSE(Value::null().isInteger());
+  EXPECT_FALSE(Value(true).isInteger());
+  EXPECT_FALSE(Value(false).isInteger());
+  EXPECT_FALSE(Value(rt, String::createFromAscii(rt, "42")).isInteger());
+  EXPECT_FALSE(Value(rt, Object(rt)).isInteger());
+
+  // NaN should return false
+  EXPECT_FALSE(Value(std::numeric_limits<double>::quiet_NaN()).isInteger());
+
+  // Infinity should return false
+  EXPECT_FALSE(Value(std::numeric_limits<double>::infinity()).isInteger());
+  EXPECT_FALSE(Value(-std::numeric_limits<double>::infinity()).isInteger());
+
+  // Non-integer numbers should return false
+  EXPECT_FALSE(Value(1.5).isInteger());
+  EXPECT_FALSE(Value(-2.3).isInteger());
+  EXPECT_FALSE(Value(0.1).isInteger());
+
+  // Integer numbers should return true
+  EXPECT_TRUE(Value(0).isInteger());
+  EXPECT_TRUE(Value(0.0).isInteger());
+  EXPECT_TRUE(Value(-0.0).isInteger());
+  EXPECT_TRUE(Value(1).isInteger());
+  EXPECT_TRUE(Value(-1).isInteger());
+  EXPECT_TRUE(Value(42).isInteger());
+  EXPECT_TRUE(Value(1000000).isInteger());
+  EXPECT_TRUE(Value(-999999).isInteger());
+
+  // Large integers that can be exactly represented as doubles
+  EXPECT_TRUE(Value(static_cast<double>(1LL << 52)).isInteger());
+  EXPECT_TRUE(Value(static_cast<double>(-(1LL << 52))).isInteger());
+}
+
 TEST_P(JSITest, EqualsTest) {
   EXPECT_TRUE(Object::strictEquals(rt, rt.global(), rt.global()));
   EXPECT_TRUE(Value::strictEquals(rt, 1, 1));