Add special handling for identifiers which consist entirely of _ characters when transformUnderscore is true to emit valid Typescript syntax

.changeset/strong-jars-fail.md

@@ -0,0 +1,5 @@
+---
+'@graphql-codegen/visitor-plugin-common': patch
+---
+
+Add special handling for identifiers that consist entirely of _ characters when transformUnderscore is true. This prevents _ values in GraphQL enums from being emitted without identifers in the resulting types.

packages/plugins/other/visitor-plugin-common/src/utils.ts

@@ -253,6 +253,13 @@ export function getBaseTypeNode(typeNode: TypeNode): NamedTypeNode {
 
 export function convertNameParts(str: string, func: (str: string) => string, removeUnderscore = false): string {
   if (removeUnderscore) {
+    // Special case: if the string is just underscores ad we're being asked to remove them, preserve it as-is.
+    // this prevents empty identifiers from making their way into the output. Yes, we want to remove underscores,
+    // but we can't when that's all the string is.
+    if (/^_+$/.test(str)) {
+      return str;
+    }
+
     return func(str);
   }
 

packages/plugins/other/visitor-plugin-common/tests/plugins-common.spec.ts

@@ -117,4 +117,15 @@ describe('convertFactory', () => {
     expect(factory('_Myname')).toBe('Myname');
     expect(factory('My_name')).toBe('Myname');
   });
+
+  it('Should handle enum values that are just underscores when transformUnderscore is true', () => {
+    const factory = convertFactory({
+      namingConvention: {
+        transformUnderscore: true,
+      },
+    });
+
+    expect(factory('_')).toBe('_');
+    expect(factory('__')).toBe('__');
+  });
 });