TS: Extract type alias relation

javascript/ql/src/semmle/javascript/TypeScript.qll

@@ -2287,6 +2287,24 @@ class EnumLiteralType extends TypeReference {
   EnumMember getEnumMember() { result = declaration }
 }
 
+/**
+ * A type that refers to a type alias.
+ */
+class TypeAliasReference extends TypeReference {
+  TypeAliasReference() {
+    type_alias(this, _)
+  }
+
+  /**
+   * Gets the type behind the type alias.
+   *
+   * For example, for `type B<T> = T[][]`, this maps the type `B<number>` to `number[][]`.
+   */
+  Type getAliasedType() {
+    type_alias(this, result)
+  }
+}
+
 /**
  * An anonymous interface type, such as `{ x: number }`.
  */

javascript/ql/src/semmlecode.javascript.dbscheme

@@ -703,6 +703,10 @@ type_property(
   varchar(900) name: string ref,
   int propertyType: @type ref);
 
+type_alias(
+  unique int aliasType: @type ref,
+  int underlyingType: @type ref);
+
 @literaltype = @stringliteraltype | @numberliteraltype | @booleanliteraltype | @bigintliteraltype;
 @type_with_literal_value = @stringliteraltype | @numberliteraltype | @bigintliteraltype;
 type_literal_value(