add buig int

src/big_int_ffi.mjs

···
+export function add(a, b) {
+  return a + b;
+}
+
+export function multiply(a, b) {
+  return a * b;
+}
+
+export function subtract(a, b) {
+  return a - b;
+}
+
+export function modulo(a, b) {
+  return a % b;
+}
+
+export function power(a, b) {
+  return a ** b;
+}
+
+export function divide(a, b) {
+  return a / b;
+}
+
+export function from(a) {
+  return BigInt(a);
+}

src/plinth/javascript/big_int.gleam

···
 pub type BigInt
 
-@external(javascript, "process", "hrtime")
+/// `+` operation
+@external(javascript, "../../big_int_ffi.mjs", "add")
+pub fn add(a: BigInt, b: BigInt) -> BigInt
+
+/// `*` operation
+@external(javascript, "../../big_int_ffi.mjs", "multiply")
+pub fn multiply(a: BigInt, b: BigInt) -> BigInt
+
+/// `-` operation
+@external(javascript, "../../big_int_ffi.mjs", "subtract")
 pub fn subtract(a: BigInt, b: BigInt) -> BigInt
+
+/// `%` operation
+@external(javascript, "../../big_int_ffi.mjs", "modulo")
+pub fn modulo(a: BigInt, b: BigInt) -> BigInt
+
+/// `**` operation
+@external(javascript, "../../big_int_ffi.mjs", "power")
+pub fn power(a: BigInt, b: BigInt) -> BigInt
+
+/// `/` operation
+@external(javascript, "../../big_int_ffi.mjs", "divide")
+pub fn divide(a: BigInt, b: BigInt) -> BigInt
+
+@external(javascript, "../../big_int_ffi.mjs", "from")
+pub fn from_int(a: Int) -> BigInt
+
+@external(javascript, "../../big_int_ffi.mjs", "from")
+pub fn from_string(a: String) -> BigInt

test/javascript/big_int_test.gleam

···
+import plinth/javascript/big_int
+import gleeunit/should
+
+pub fn add_test() {
+  big_int.from_int(1)
+  |> should.equal(big_int.from_string("1"))
+}