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/base-types-visitor.ts

@@ -495,6 +495,8 @@ export interface RawTypesConfig extends RawConfig {
   directiveArgumentAndInputFieldMappingTypeSuffix?: string;
 }
 
+const onlyUnderscoresPattern = /^_+$/;
+
 export class BaseTypesVisitor<
   TRawConfig extends RawTypesConfig = RawTypesConfig,
   TPluginConfig extends ParsedTypesConfig = ParsedTypesConfig
@@ -915,7 +917,9 @@ export class BaseTypesVisitor<
         const optionName = this.makeValidEnumIdentifier(
           this.convertName(enumOption, {
             useTypesPrefix: false,
-            transformUnderscore: true,
+            // We can only strip out the underscores if the value contains other
+            // characters. Otherwise we'll generate syntactically invalid code.
+            transformUnderscore: !onlyUnderscoresPattern.test(enumOption.name as any),
           })
         );
         const comment = this.getNodeComment(enumOption);

packages/plugins/typescript/typescript/tests/typescript.spec.ts

@@ -193,7 +193,7 @@ describe('TypeScript', () => {
       }`);
     });
 
-    it('Should removed underscore from enum values', async () => {
+    it('Should remove underscore from enum values', async () => {
       const schema = buildSchema(/* GraphQL */ `
         enum MyEnum {
           A_B_C
@@ -213,6 +213,24 @@ describe('TypeScript', () => {
       }`);
     });
 
+    it('Should leave underscores in enum values when the value is only underscores', async () => {
+      const schema = buildSchema(/* GraphQL */ `
+        enum MyEnum {
+          _
+          __
+          _TEST
+        }
+      `);
+      const result = await plugin(schema, [], {}, { outputFile: '' });
+
+      expect(result.content).toBeSimilarStringTo(`
+      export enum MyEnum {
+        _ = '_',
+        __ = '__',
+        Test = '_TEST'
+      }`);
+    });
+
     it('Should work with enum as const', async () => {
       const schema = buildSchema(/* GraphQL */ `
         enum MyEnum {