feat: add option to add __typename to interfaces

packages/plugins/other/visitor-plugin-common/src/base-resolvers-visitor.ts

@@ -1071,7 +1071,9 @@ export class BaseResolversVisitor<
     relevantFields: ReturnType<typeof this.getRelevantFieldsToOmit>
   ): string {
     this._globalDeclarations.add(OMIT_TYPE);
-    return `Omit<${typeName}, ${relevantFields.map(f => `'${f.fieldName}'`).join(' | ')}> & { ${relevantFields
+    return `Omit<${typeName}, ${relevantFields
+      .map(f => `'${f.fieldName}'`)
+      .join(this.typeUnionOperator)}> & { ${relevantFields
       .map(f => `${f.fieldName}${f.addOptionalSign ? '?' : ''}: ${f.replaceWithType}`)
       .join(', ')} }`;
   }
@@ -1222,7 +1224,7 @@ export class BaseResolversVisitor<
         ? 'never'
         : members.length > 1
         ? `\n    | ${members.map(m => m.replace(/\n/g, '\n  ')).join('\n    | ')}\n  `
-        : members.join(' | ');
+        : members.join(this.typeUnionOperator);
     return result;
   }
 
@@ -1783,7 +1785,7 @@ export class BaseResolversVisitor<
 
   protected applyRequireFields(argsType: string, fields: InputValueDefinitionNode[]): string {
     this._globalDeclarations.add(REQUIRE_FIELDS_TYPE);
-    return `RequireFields<${argsType}, ${fields.map(f => `'${f.name.value}'`).join(' | ')}>`;
+    return `RequireFields<${argsType}, ${fields.map(f => `'${f.name.value}'`).join(this.typeUnionOperator)}>`;
   }
 
   protected applyOptionalFields(argsType: string, _fields: readonly InputValueDefinitionNode[]): string {
@@ -1875,7 +1877,7 @@ export class BaseResolversVisitor<
     const possibleTypes = originalNode.types
       .map(node => node.name.value)
       .map(f => `'${f}'`)
-      .join(' | ');
+      .join(this.typeUnionOperator);
 
     this._collectedResolvers[node.name.value] = {
       typename: name + '<ContextType>',
@@ -2039,7 +2041,7 @@ export class BaseResolversVisitor<
       typeName,
     });
 
-    const possibleTypes = implementingTypes.map(name => `'${name}'`).join(' | ') || 'null';
+    const possibleTypes = implementingTypes.map(name => `'${name}'`).join(this.typeUnionOperator) || 'null';
 
     // An Interface has __resolveType resolver, and no other fields.
     const blockFields: string[] = [

packages/plugins/other/visitor-plugin-common/src/base-types-visitor.ts

@@ -5,6 +5,7 @@ import {
   EnumValueDefinitionNode,
   FieldDefinitionNode,
   GraphQLEnumType,
+  GraphQLInterfaceType,
   GraphQLSchema,
   InputObjectTypeDefinitionNode,
   InputValueDefinitionNode,
@@ -713,7 +714,7 @@ export class BaseTypesVisitor<
     const originalNode = parent[key] as UnionTypeDefinitionNode;
     const possibleTypes = originalNode.types
       .map(t => (this.scalars[t.name.value] ? this._getScalar(t.name.value, 'output') : this.convertName(t)))
-      .join(' | ');
+      .join(this.typeUnionOperator);
 
     return new DeclarationBlock(this._declarationBlockConfig)
       .export()
@@ -732,22 +733,20 @@ export class BaseTypesVisitor<
     block.withBlock(this.mergeAllFields(fields, interfaces.length > 0));
   }
 
+  getTypenameField(type: DeclarationKind, typeNames: string[]) {
+    const optionalTypename = this.config.nonOptionalTypename ? '__typename' : '__typename?';
+    return `${this.config.immutableTypes ? 'readonly ' : ''}${optionalTypename}: ${typeNames
+      .map(typeName => `'${typeName}'`)
+      .join(this.typeUnionOperator)}${this.getPunctuation(type)}`;
+  }
+
   getObjectTypeDeclarationBlock(
     node: ObjectTypeDefinitionNode,
     originalNode: ObjectTypeDefinitionNode
   ): DeclarationBlock {
-    const optionalTypename = this.config.nonOptionalTypename ? '__typename' : '__typename?';
     const { type, interface: interfacesType } = this._parsedConfig.declarationKind;
     const allFields = [
-      ...(this.config.addTypename
-        ? [
-            indent(
-              `${this.config.immutableTypes ? 'readonly ' : ''}${optionalTypename}: '${
-                node.name.value
-              }'${this.getPunctuation(type)}`
-            ),
-          ]
-        : []),
+      ...(this.config.addTypename ? [indent(this.getTypenameField(type, [node.name.value]))] : []),
       ...node.fields,
     ] as string[];
     const interfacesNames = originalNode.interfaces ? originalNode.interfaces.map(i => this.convertName(i)) : [];
@@ -790,13 +789,28 @@ export class BaseTypesVisitor<
     node: InterfaceTypeDefinitionNode,
     _originalNode: InterfaceTypeDefinitionNode
   ): DeclarationBlock {
+    const { type, interface: interfacesType } = this._parsedConfig.declarationKind;
     const declarationBlock = new DeclarationBlock(this._declarationBlockConfig)
       .export()
-      .asKind(this._parsedConfig.declarationKind.interface)
+      .asKind(interfacesType)
       .withName(this.convertName(node))
       .withComment(node.description?.value);
 
-    return declarationBlock.withBlock(node.fields.join('\n'));
+    const schemaType = this._schema.getType(node.name.value);
+    const { objects: concreteTypes } = this._schema.getImplementations(schemaType as GraphQLInterfaceType);
+    const allFields = [
+      ...node.fields,
+      ...(this.config.addTypenameToInterfaces && concreteTypes.length > 0
+        ? [
+            this.getTypenameField(
+              type,
+              concreteTypes.map(({ name }) => name)
+            ),
+          ]
+        : []),
+    ];
+
+    return declarationBlock.withBlock(allFields.join('\n'));
   }
 
   InterfaceTypeDefinition(node: InterfaceTypeDefinitionNode, key: number | string, parent: any): string {

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

@@ -28,6 +28,7 @@ export interface ParsedConfig {
   typesPrefix: string;
   typesSuffix: string;
   addTypename: boolean;
+  addTypenameToInterfaces: boolean;
   nonOptionalTypename: boolean;
   extractAllFieldsToTypes: boolean;
   externalFragments: LoadedFragment[];
@@ -283,6 +284,28 @@ export interface RawConfig {
    * ```
    */
   skipTypename?: boolean;
+  /**
+   * @description Similarly to the `addTypename` option, if true, a `__typename` field will be added to type definitions resulting from interface declarations.
+   *
+   * @exampleMarkdown
+   * ```ts filename="codegen.ts"
+   *  import type { CodegenConfig } from '@graphql-codegen/cli';
+   *
+   *  const config: CodegenConfig = {
+   *    // ...
+   *    generates: {
+   *      'path/to/file': {
+   *        // plugins...
+   *        config: {
+   *          addTypenameToInterfaces: true
+   *        },
+   *      },
+   *    },
+   *  };
+   *  export default config;
+   * ```
+   */
+  addTypenameToInterfaces?: boolean;
   /**
    * @default false
    * @description Automatically adds `__typename` field to the generated types, even when they are not specified
@@ -416,6 +439,7 @@ export class BaseVisitor<TRawConfig extends RawConfig = RawConfig, TPluginConfig
       externalFragments: rawConfig.externalFragments || [],
       fragmentImports: rawConfig.fragmentImports || [],
       addTypename: !rawConfig.skipTypename,
+      addTypenameToInterfaces: !rawConfig.skipTypename && rawConfig.addTypenameToInterfaces,
       nonOptionalTypename: !!rawConfig.nonOptionalTypename,
       useTypeImports: !!rawConfig.useTypeImports,
       allowEnumStringTypes: !!rawConfig.allowEnumStringTypes,
@@ -451,6 +475,10 @@ export class BaseVisitor<TRawConfig extends RawConfig = RawConfig, TPluginConfig
     return this._parsedConfig;
   }
 
+  get typeUnionOperator() {
+    return ' | ';
+  }
+
   public convertName(node: ASTNode | string, options?: BaseVisitorConvertOptions & ConvertOptions): string {
     const useTypesPrefix = typeof options?.useTypesPrefix === 'boolean' ? options.useTypesPrefix : true;
     const useTypesSuffix = typeof options?.useTypesSuffix === 'boolean' ? options.useTypesSuffix : true;

packages/plugins/typescript/typescript/src/visitor.ts

@@ -125,7 +125,7 @@ export class TsVisitor<
       }
 
       if (implementingTypes.length > 0) {
-        return implementingTypes.join(' | ');
+        return implementingTypes.join(this.typeUnionOperator);
       }
     }
 
@@ -255,7 +255,7 @@ export class TsVisitor<
     const possibleTypes = originalNode.types
       .map(t => (this.scalars[t.name.value] ? this._getScalar(t.name.value, 'output') : this.convertName(t)))
       .concat(...withFutureAddedValue)
-      .join(' | ');
+      .join(this.typeUnionOperator);
 
     return new DeclarationBlock(this._declarationBlockConfig)
       .export()

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

@@ -3950,6 +3950,37 @@ describe('TypeScript', () => {
     `);
   });
 
+  it('should union concrete type names in interface __typename field - issue #10522', async () => {
+    debugger;
+    const testSchema = buildSchema(/* GraphQL */ `
+      interface TopLevel {
+        topLevelField: Boolean
+      }
+
+      type OneImplementation implements TopLevel {
+        topLevelField: Boolean
+        implementationField: String
+      }
+
+      type AnotherImplementation implements TopLevel {
+        topLevelField: Boolean
+        anotherImplementationField: Int
+      }
+    `);
+    const output = await plugin(
+      testSchema,
+      [],
+      {
+        addTypenameToInterfaces: true,
+      },
+      { outputFile: 'graphql.ts' }
+    );
+
+    expect(output.content).toBeSimilarStringTo(`
+      __typename?: 'OneImplementation' | 'AnotherImplementation'
+    `);
+  });
+
   it('should use implementing types as node type - issue #5126', async () => {
     const testSchema = buildSchema(/* GraphQL */ `
       type Matrix {