[Typescript] enum as const: add support const assertions from T… (#3569)

* Typescript plugin: enumAsConst feature

* add type gen

Author: DragorWW

Diff for: docs/generated-config/typescript.md

@@ -60,6 +60,21 @@ path/to/file.ts:
    enumsAsTypes: true
 ```
 
+### enumsAsConst (`boolean`, default value: `false`)
+
+Generates enum as TypeScript `const assertions` instead of `enum`. This can even be used to enable enum-like patterns in plain JavaScript code if you choose not to use TypeScript’s enum construct.
+
+#### Usage Example
+
+```yml
+generates:
+path/to/file.ts:
+ plugins:
+   - typescript
+ config:
+   enumsAsConst: true
+```
+
 ### immutableTypes (`boolean`, default value: `false`)
 
 Generates immutable types by adding `readonly` to properties and uses `ReadonlyArray`.
@@ -106,4 +121,4 @@ path/to/file.ts:
    - typescript
  config:
    noExport: true
-```
+```

Diff for: packages/plugins/typescript/typescript/src/config.ts

@@ -68,6 +68,23 @@ export interface TypeScriptPluginConfig extends RawTypesConfig {
    * ```
    */
   enumsAsTypes?: boolean;
+  /**
+   * @name enumsAsConst
+   * @type boolean
+   * @description Generates enum as TypeScript `const assertions` instead of `enum`. This can even be used to enable enum-like patterns in plain JavaScript code if you choose not to use TypeScript’s enum construct.
+   * @default false
+   *
+   * @example
+   * ```yml
+   * generates:
+   * path/to/file.ts:
+   *  plugins:
+   *    - typescript
+   *  config:
+   *    enumsAsConst: true
+   * ```
+   */
+  enumsAsConst?: boolean;
   /**
    * @name fieldWrapperValue
    * @type string

Diff for: packages/plugins/typescript/typescript/src/visitor.ts

@@ -10,6 +10,7 @@ export interface TypeScriptPluginParsedConfig extends ParsedTypesConfig {
   avoidOptionals: AvoidOptionalsConfig;
   constEnums: boolean;
   enumsAsTypes: boolean;
+  enumsAsConst: boolean;
   fieldWrapperValue: string;
   immutableTypes: boolean;
   maybeValue: string;
@@ -26,6 +27,7 @@ export class TsVisitor<TRawConfig extends TypeScriptPluginConfig = TypeScriptPlu
       fieldWrapperValue: getConfigValue(pluginConfig.fieldWrapperValue, 'T'),
       constEnums: getConfigValue(pluginConfig.constEnums, false),
       enumsAsTypes: getConfigValue(pluginConfig.enumsAsTypes, false),
+      enumsAsConst: getConfigValue(pluginConfig.enumsAsConst, false),
       immutableTypes: getConfigValue(pluginConfig.immutableTypes, false),
       wrapFieldDefinitions: getConfigValue(pluginConfig.wrapFieldDefinitions, false),
       ...(additionalConfig || {}),
@@ -133,14 +135,45 @@ export class TsVisitor<TRawConfig extends TypeScriptPluginConfig = TypeScriptPlu
               })
               .join(' |\n')
         ).string;
-    } else {
-      return new DeclarationBlock(this._declarationBlockConfig)
+    }
+
+    if (this.config.enumsAsConst) {
+      const typeName = `export type ${enumTypeName} = typeof ${enumTypeName}[keyof typeof ${enumTypeName}];`;
+      const enumAsConst = new DeclarationBlock({
+        ...this._declarationBlockConfig,
+        blockTransformer: block => {
+          return block + ' as const';
+        },
+      })
         .export()
-        .asKind(this.config.constEnums ? 'const enum' : 'enum')
+        .asKind('const')
         .withName(enumTypeName)
         .withComment((node.description as any) as string)
-        .withBlock(this.buildEnumValuesBlock(enumName, node.values)).string;
+        .withBlock(
+          node.values
+            .map(enumOption => {
+              const optionName = this.convertName(enumOption, { useTypesPrefix: false, transformUnderscore: true });
+              const comment = transformComment((enumOption.description as any) as string, 1);
+              let enumValue: string | number = enumOption.name as any;
+
+              if (this.config.enumValues[enumName] && this.config.enumValues[enumName].mappedValues && typeof this.config.enumValues[enumName].mappedValues[enumValue] !== 'undefined') {
+                enumValue = this.config.enumValues[enumName].mappedValues[enumValue];
+              }
+
+              return comment + indent(`${optionName}: ${wrapWithSingleQuotes(enumValue)}`);
+            })
+            .join(',\n')
+        ).string;
+
+      return [enumAsConst, typeName].join('\n');
     }
+
+    return new DeclarationBlock(this._declarationBlockConfig)
+      .export()
+      .asKind(this.config.constEnums ? 'const enum' : 'enum')
+      .withName(enumTypeName)
+      .withComment((node.description as any) as string)
+      .withBlock(this.buildEnumValuesBlock(enumName, node.values)).string;
   }
 
   protected getPunctuation(declarationKind: DeclarationKind): string {

Diff for: packages/plugins/typescript/typescript/tests/typescript.spec.ts

@@ -202,6 +202,27 @@ describe('TypeScript', () => {
       }`);
     });
 
+    it('Should work with enum as const', async () => {
+      const schema = buildSchema(/* GraphQL */ `
+        enum MyEnum {
+          A_B_C
+          X_Y_Z
+          _TEST
+          My_Value
+        }
+      `);
+      const result = (await plugin(schema, [], { enumsAsConst: true }, { outputFile: '' })) as Types.ComplexPluginOutput;
+
+      expect(result.content).toBeSimilarStringTo(`
+      export const MyEnum = {
+        ABC: 'A_B_C',
+        XYZ: 'X_Y_Z',
+        Test: '_TEST',
+        MyValue: 'My_Value'
+      } as const;
+      export type MyEnum = typeof MyEnum[keyof typeof MyEnum];`);
+    });
+
     it('Should work with enum and enum values (enumsAsTypes)', async () => {
       const schema = buildSchema(/* GraphQL */ `
         "custom enum"