Primitives (number, boolean, bigint)

Primitive datatypes like number, boolean or bigint can use platform-native datatypes directly. For example, a JS number is always a 64-bit double in C++, a Double in Swift, and a Double in Kotlin.

TypeScript

interface Math extends HybridObject {
  add(a: number, b: number): number
}

Swift

class HybridMath: HybridMathSpec {
  func add(a: Double, b: Double) -> Double
}

Kotlin

class HybridMath: HybridMathSpec() {
  fun add(a: Double, b: Double): Double
}

C++

class HybridMath : public HybridMathSpec {
  double add(double a, double b);
}

Primitives are very efficient and can be passed with little to no overhead, especially between C++ and Swift, and C++ and Kotlin.

bigint's actual type

A bigint is actually a variable-size type. While it is bridged to a 64-bit Integer (-2⁶³ … 2⁶³−1), it can theoretically be larger than that.

Since there is no built-in equivalent of bigint in C++/Swift/Kotlin, you'd need to stringify the bigint value on the JS side and parse it from a string to your big number library of choice on the native side again in cases where you really need big numbers.