lib: implement interface converter in webidl

lib/internal/webidl.js

@@ -12,6 +12,7 @@ const {
   NumberMAX_SAFE_INTEGER,
   NumberMIN_SAFE_INTEGER,
   ObjectAssign,
+  ObjectPrototypeIsPrototypeOf,
   SafeSet,
   String,
   SymbolIterator,
@@ -20,6 +21,7 @@ const {
 
 const {
   codes: {
+    ERR_INVALID_ARG_TYPE,
     ERR_INVALID_ARG_VALUE,
   },
 } = require('internal/errors');
@@ -275,20 +277,34 @@ function createSequenceConverter(converter) {
       const val = converter(res.value, {
         __proto__: null,
         ...opts,
-        context: `${opts.context}, index ${array.length}`,
+        context: `${opts.context}[${array.length}]`,
       });
       ArrayPrototypePush(array, val);
     };
     return array;
   };
 }
 
+// https://webidl.spec.whatwg.org/#js-interface
+function createInterfaceConverter(name, I) {
+  return (V, opts = kEmptyObject) => {
+    // 1. If V implements I, then return the IDL interface type value that
+    //    represents a reference to that platform object.
+    if (ObjectPrototypeIsPrototypeOf(I, V)) return V;
+    // 2. Throw a TypeError.
+    throw new ERR_INVALID_ARG_TYPE(
+      typeof opts.context === 'string' ? opts.context : 'value', name, V,
+    );
+  };
+}
+
 
 module.exports = {
   type,
   converters,
   convertToInt,
   createEnumConverter,
+  createInterfaceConverter,
   createSequenceConverter,
   evenRound,
   makeException,