Advanced features

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diff --git a/src/content/chapter5_advance_features/lesson04_externals/code.gleam b/src/content/chapter5_advance_features/lesson04_externals/code.gleam
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+import gleam/io
+
+// A type with no Gleam constructors
+pub type DateTime
+
+// An external function that creates an instance of the type
+@external(javascript, "./my_package_ffi.mjs", "now")
+pub fn now() -> DateTime
+
+// The `now` function in `./my_package_ffi.mjs` looks like this:
+// external function now() {
+//   return new Date();
+// }
+
+pub fn main() {
+  io.debug(now())
+}
+