diff --git a/libraries/tools/idl2k/pom.xml b/libraries/tools/idl2k/pom.xml
index fcdf0bd23a5..8fa3ec7a00f 100644
--- a/libraries/tools/idl2k/pom.xml
+++ b/libraries/tools/idl2k/pom.xml
@@ -50,6 +50,7 @@
                 <version>${antlr.version}</version>
                 <executions>
                     <execution>
+                        <id>antlr</id>
                         <goals>
                             <goal>antlr4</goal>
                         </goals>
diff --git a/libraries/tools/idl2k/src/main/antlr4/org/antlr/webidl/WebIDL.g4 b/libraries/tools/idl2k/src/main/antlr4/org/antlr/webidl/WebIDL.g4
index 76839736132..b46a2d632b3 100644
--- a/libraries/tools/idl2k/src/main/antlr4/org/antlr/webidl/WebIDL.g4
+++ b/libraries/tools/idl2k/src/main/antlr4/org/antlr/webidl/WebIDL.g4
@@ -465,7 +465,7 @@ nonAnyType
 	| 'DOMString' typeSuffix
 	| 'USVString' typeSuffix
 	| IDENTIFIER_WEBIDL typeSuffix
-	| 'sequence' '<' type '>' null_
+	| sequenceType null_
 	| 'object' typeSuffix
 	| 'Date' typeSuffix
 	| 'RegExp' typeSuffix
@@ -506,6 +506,11 @@ integerType
 	| 'long'+
 ;
 
+sequenceType
+    : 'sequence' '<' type '>'
+    | 'FrozenArray' '<' type '>'
+;
+
 promiseType
 	: 'Promise' '<' returnType '>'
 ;
diff --git a/libraries/tools/idl2k/src/main/kotlin/idl.kt b/libraries/tools/idl2k/src/main/kotlin/idl.kt
index a74d9db069c..db227c1038d 100644
--- a/libraries/tools/idl2k/src/main/kotlin/idl.kt
+++ b/libraries/tools/idl2k/src/main/kotlin/idl.kt
@@ -20,6 +20,7 @@ import org.antlr.v4.runtime.CharStream
 import org.antlr.v4.runtime.CommonTokenStream
 import org.antlr.v4.runtime.ParserRuleContext
 import org.antlr.v4.runtime.tree.ParseTree
+import org.antlr.v4.runtime.tree.RuleNode
 import org.antlr.v4.runtime.tree.TerminalNode
 import org.antlr.webidl.WebIDLBaseVisitor
 import org.antlr.webidl.WebIDLLexer
@@ -130,11 +131,24 @@ class UnionTypeVisitor(val namespace: String) : WebIDLBaseVisitor<List<Type>>()
 
 class TypeVisitor(val namespace: String) : WebIDLBaseVisitor<Type>() {
     private var type: Type = AnyType()
+    private var awaitingSimpleType = false
 
     override fun defaultResult() = type
 
+    override fun visitType(ctx: TypeContext?): Type {
+        type = super.visitType(ctx)
+        return type
+    }
+
+    override fun visitReturnType(ctx: ReturnTypeContext?): Type {
+        awaitingSimpleType = true
+        type = super.visitReturnType(ctx)
+        return type
+    }
+
     override fun visitNonAnyType(ctx: WebIDLParser.NonAnyTypeContext): Type {
-        type = SimpleType(ctx.text, false)
+        awaitingSimpleType = true
+        type = super.visitNonAnyType(ctx)
         return type
     }
 
@@ -143,19 +157,62 @@ class TypeVisitor(val namespace: String) : WebIDLBaseVisitor<Type>() {
         return type
     }
 
+    override fun visitPromiseType(ctx: PromiseTypeContext): Type {
+        type = PromiseType(TypeVisitor(namespace).visitChildren(ctx), false)
+        return type
+    }
+
+    override fun visitSequenceType(ctx: SequenceTypeContext): Type {
+        val mutable = ctx.getChild(0).text == "sequence"
+        type = ArrayType(TypeVisitor(namespace).visitChildren(ctx), mutable = mutable, nullable = false)
+        return type
+    }
+
     override fun visitTypeSuffix(ctx: TypeSuffixContext): Type {
         when (ctx.text?.trim()) {
             "?" -> type = type.toNullable()
-            "[]" -> type = ArrayType(type, false)
-            "[]?" -> type = ArrayType(type, true)
-            "?[]" -> type = ArrayType(type.toNullable(), false)
+            "[]" -> type = ArrayType(type, mutable = true, nullable = false)
+            "[]?" -> type = ArrayType(type, mutable = true, nullable = false)
+            "?[]" -> type = ArrayType(type.toNullable(), mutable = true, nullable = false)
         }
 
         return type
     }
 
+    override fun visitNull_(ctx: Null_Context): Type {
+        if (ctx.text?.trim() == "?") {
+            type = type.toNullable()
+        }
+        return type
+    }
+
     override fun visitTerminal(node: TerminalNode): Type {
-        type = SimpleType(node.text, false)
+        if (awaitingSimpleType) {
+            type = SimpleType(node.text, false)
+            awaitingSimpleType = false
+        }
+        return type
+    }
+
+    override fun visitUnsignedIntegerType(ctx: UnsignedIntegerTypeContext): Type {
+        awaitingSimpleType = false
+        type = super.visitUnsignedIntegerType(ctx)
+        return type
+    }
+
+    override fun visitUnrestrictedFloatType(ctx: UnrestrictedFloatTypeContext): Type {
+        awaitingSimpleType = false
+        type = super.visitUnrestrictedFloatType(ctx)
+        return type
+    }
+
+    override fun visitFloatType(ctx: FloatTypeContext): Type {
+        type = SimpleType(ctx.text, false)
+        return type
+    }
+
+    override fun visitIntegerType(ctx: IntegerTypeContext): Type {
+        type = SimpleType(ctx.text, false)
         return type
     }
 }
diff --git a/libraries/tools/idl2k/src/main/kotlin/typeMappings.kt b/libraries/tools/idl2k/src/main/kotlin/typeMappings.kt
index 9c75c0edb6e..d0205e114af 100644
--- a/libraries/tools/idl2k/src/main/kotlin/typeMappings.kt
+++ b/libraries/tools/idl2k/src/main/kotlin/typeMappings.kt
@@ -63,6 +63,8 @@ fun Type.dynamicIfUnknownType(allTypes: Set<String>, standardTypes: Set<Type> =
     this is ArrayType -> copy(memberType = this.memberType.dynamicIfUnknownType(allTypes, standardTypes))
     this is UnionType -> if (this.name !in allTypes) DynamicType else this
     this is FunctionType -> copy(returnType = returnType.dynamicIfUnknownType(allTypes, standardTypes), parameterTypes = parameterTypes.map { it.copy(type = it.type.dynamicIfUnknownType(allTypes, standardTypes)) })
+    this is PromiseType ->
+        copy(valueType = valueType.dynamicIfUnknownType(allTypes, standardTypes))
 
     else -> DynamicType
 }
@@ -74,19 +76,14 @@ internal fun mapType(repository: Repository, type: Type): Type = when (type) {
         val typeName = type.type
         when {
             typeName in typeMapper -> typeMapper[typeName]!!.withNullability(type.nullable)
-            typeName.endsWith("?") -> mapType(repository, SimpleType(typeName.removeSuffix("?"), false)).toNullable()
-            typeName.endsWith("[]") -> ArrayType(memberType = mapType(repository, SimpleType(typeName.removeSuffix("[]"), false)), nullable = type.nullable)
-            typeName.startsWith("unrestricted") -> mapType(repository, SimpleType(typeName.removePrefix("unrestricted"), false))
-            typeName.startsWith("sequence<") -> ArrayType(mapType(repository, SimpleType(typeName.removePrefix("sequence<").removeSuffix(">").trim(), false)), type.nullable)
-            typeName == "sequence" -> ArrayType(DynamicType, type.nullable)
             typeName in repository.interfaces -> type
             typeName in repository.typeDefs -> mapTypedef(repository, type)
             typeName in repository.enums -> SimpleType("String", type.nullable)
-            typeName.startsWith("Promise<") -> DynamicType
 
             else -> type
         }
     }
+    is PromiseType -> type.copy(valueType = mapType(repository, type.valueType))
     is ArrayType -> type.copy(memberType = mapType(repository, type.memberType))
     is UnionType -> UnionType(type.namespace, type.memberTypes.map { mt -> mapType(repository, mt) }, type.nullable).toSingleTypeIfPossible()
     is FunctionType -> type.copy(
@@ -94,7 +91,10 @@ internal fun mapType(repository: Repository, type: Type): Type = when (type) {
             returnType = mapType(repository, type.returnType).dynamicIfAnyType(),
             parameterTypes = type.parameterTypes.takeWhile { !it.vararg }.map { it.copy(type = mapType(repository, it.type)) }
     )
-    else -> type
+
+    is AnyType,
+    is UnitType,
+    is DynamicType -> type
 }
 
 private fun mapTypedef(repository: Repository, type: SimpleType): Type {
diff --git a/libraries/tools/idl2k/src/main/kotlin/types.kt b/libraries/tools/idl2k/src/main/kotlin/types.kt
index 3e552d38689..ee7163a79b7 100644
--- a/libraries/tools/idl2k/src/main/kotlin/types.kt
+++ b/libraries/tools/idl2k/src/main/kotlin/types.kt
@@ -2,40 +2,43 @@ package org.jetbrains.idl2k
 
 import java.util.*
 
-interface Type {
-    val nullable: Boolean
-    fun render(): String
+sealed class Type {
+    abstract val nullable: Boolean
+    abstract fun render(): String
 }
 
 private fun String.appendNullabilitySuffix(type: Type) = if (type.nullable) "$this?" else this
 
-object UnitType : Type {
+object UnitType : Type() {
     override val nullable: Boolean
         get() = false
 
     override fun render() = "Unit"
 }
-object DynamicType : Type {
+object DynamicType : Type() {
     override val nullable: Boolean
         get() = false
 
     override fun render() = "dynamic"
 }
-data class AnyType(override val nullable: Boolean = true) : Type {
+data class AnyType(override val nullable: Boolean = true) : Type() {
     override fun render() = "Any".appendNullabilitySuffix(this)
 }
-data class SimpleType(val type: String, override val nullable: Boolean) : Type {
+data class SimpleType(val type: String, override val nullable: Boolean) : Type() {
     override fun render() = type.appendNullabilitySuffix(this)
 }
-data class FunctionType(val parameterTypes : List<Attribute>, val returnType : Type, override val nullable: Boolean) : Type {
+data class FunctionType(val parameterTypes : List<Attribute>, val returnType : Type, override val nullable: Boolean) : Type() {
     override fun render() = if (nullable) "(${renderImpl()})?" else renderImpl()
     private fun renderImpl() = "(${parameterTypes.joinToString(", ") { it.type.render() }}) -> ${returnType.render()}"
 }
+data class PromiseType(val valueType: Type, override val nullable: Boolean) : Type() {
+    override fun render() = "Promise<${valueType.render()}>".appendNullabilitySuffix(this)
+}
 
 val FunctionType.arity : Int
     get() = parameterTypes.size
 
-class UnionType(val namespace: String, types: Collection<Type>, override val nullable: Boolean) : Type {
+class UnionType(val namespace: String, types: Collection<Type>, override val nullable: Boolean) : Type() {
     val memberTypes: Set<Type> = LinkedHashSet(types.sortedBy { it.toString() })
     val name = "Union${this.memberTypes.map { it.render() }.joinToString("Or")}"
 
@@ -52,8 +55,8 @@ class UnionType(val namespace: String, types: Collection<Type>, override val nul
 
 fun UnionType.toSingleTypeIfPossible() = if (this.memberTypes.size == 1) this.memberTypes.single().withNullability(nullable) else this
 
-data class ArrayType(val memberType: Type, override val nullable: Boolean) : Type {
-    override fun render(): String = "Array<${memberType.render()}>".appendNullabilitySuffix(this)
+data class ArrayType(val memberType: Type, val mutable: Boolean, override val nullable: Boolean) : Type() {
+    override fun render(): String = "Array<${if (mutable) "" else "out "}${memberType.render()}>".appendNullabilitySuffix(this)
 }
 
 @Suppress("UNCHECKED_CAST")
@@ -66,6 +69,7 @@ private fun <T: Type> T.copyWithNullability(nullable: Boolean): T = when (this)
     is FunctionType -> this.copy(nullable = nullable)
     is UnionType -> this.copy(types = this.memberTypes, nullable = nullable)
     is ArrayType -> this.copy(nullable = nullable)
+    is PromiseType -> this.copy(nullable = nullable)
     else -> throw UnsupportedOperationException()
 } as T