adding required methods and functions for "__typename must be const" change

src/Errors.ts

@@ -156,6 +156,22 @@ export function typeNameInitializeNotString() {
   return `Expected \`__typename\` property initializer to be a string literal. For example: \`__typename = "MyType"\` or \`__typename: "MyType";\`. ${TYPENAME_CONTEXT}`;
 }
 
+export function typeNameInitializeNotExpression() {
+  return `Expected \`__typename\` property initializer to be an expression with a const assertion. For example: \`__typename = "MyType" as const\`. ${TYPENAME_CONTEXT}`;
+}
+
+export function typeNameTypeNotReferenceNode() {
+  return `Expected \`__typename\` property type to be a TypeReferenceNode. ${TYPENAME_CONTEXT}`;
+}
+
+export function typeNameTypeNameNotIdentifier() {
+  return `Expected \`__typename\` property type name to be an Identifier. ${TYPENAME_CONTEXT}`;
+}
+
+export function typeNameTypeNameNotConst() {
+  return `Expected \`__typename\` property type name to be "const". ${TYPENAME_CONTEXT}`;
+}
+
 export function typeNameInitializerWrong(expected: string, actual: string) {
   return `Expected \`__typename\` property initializer to be \`"${expected}"\`, found \`"${actual}"\`. ${TYPENAME_CONTEXT}`;
 }

src/Extractor.ts

@@ -740,18 +740,39 @@ class Extractor {
       return false;
     }
 
-    if (!ts.isStringLiteral(node.initializer)) {
-      this.report(node.initializer, E.typeNameInitializeNotString());
+    if (!ts.isAsExpression(node.initializer)) {
+      this.report(node.initializer, E.typeNameInitializeNotExpression())
       return false;
     }
 
-    if (node.initializer.text !== expectedName) {
+    if (!ts.isStringLiteral(node.initializer.expression)) {
+      this.report(node.initializer.expression, E.typeNameInitializeNotString());
+      return false
+    }
+
+    if (node.initializer.expression.text !== expectedName) {
       this.report(
         node.initializer,
-        E.typeNameInitializerWrong(expectedName, node.initializer.text),
+        E.typeNameInitializerWrong(expectedName, node.initializer.expression.text),
       );
+      return false
+    }
+
+    if (!ts.isTypeReferenceNode(node.initializer.type)) {
+      this.report(node.initializer.type, E.typeNameTypeNotReferenceNode());
       return false;
     }
+
+    if (!ts.isIdentifier(node.initializer.type.typeName)) {
+      this.report(node.initializer.type.typeName, E.typeNameTypeNameNotIdentifier());
+      return false;
+    }
+
+    if (node.initializer.type.typeName.escapedText !== "const") {
+      this.report(node.initializer.type.typeName, E.typeNameTypeNameNotConst());
+      return false;
+    }
+
     return true;
   }