diff --git a/book-src/tour/custom-types.md b/book-src/tour/custom-types.md
index 5a03a13a..20a96798 100644
--- a/book-src/tour/custom-types.md
+++ b/book-src/tour/custom-types.md
@@ -250,3 +250,25 @@ LoggedIn("Kim")
 guest,
 {logged_in, <<"Kim">>}.
 ```
+
+## JavaScript interop
+
+When compiling to JavaScript, each constructor becomes a class of the same name.
+Each of the generated JavaScript classes has a constructor with the same
+arguments as the original type constructor.
+
+```gleam
+// Gleam
+Guest
+LoggedIn("Kim")
+Cat(name: "Jinx", cuteness: 2002)
+```
+
+```javascript
+// JavaScript
+import { Guest, LoggedIn, Cat } from "my_module.mjs";
+
+new Guest();
+new LoggedIn("Kim");
+new Cat("Jinx", 2002);
+```