interface over type so we can extend

Author: SimenB

packages/babel-jest/src/index.ts

@@ -23,14 +23,15 @@ const THIS_FILE = fs.readFileSync(__filename);
 const jestPresetPath = require.resolve('babel-preset-jest');
 const babelIstanbulPlugin = require.resolve('babel-plugin-istanbul');
 
-// Make it non-optional
-type TransformerWithFactory = Transformer & {
-  createTransformer: (options?: TransformOptions) => TransformerWithFactory;
-};
+// Narrow down the types
+interface BabelJestTransformer extends Transformer {
+  canInstrument: true;
+  createTransformer: (options?: TransformOptions) => BabelJestTransformer;
+}
 
 const createTransformer = (
   options: TransformOptions = {},
-): TransformerWithFactory => {
+): BabelJestTransformer => {
   options = {
     ...options,
     // @ts-ignore: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/33118
@@ -64,7 +65,7 @@ const createTransformer = (
     return babelConfig;
   }
 
-  const transformer: TransformerWithFactory = {
+  const transformer: BabelJestTransformer = {
     canInstrument: true,
     createTransformer,
     getCacheKey(

packages/jest-transform/src/types.ts

@@ -51,7 +51,7 @@ export type CacheKeyOptions = {
   rootDir: string;
 };
 
-export type Transformer = {
+export interface Transformer {
   canInstrument?: boolean;
   createTransformer?: (options?: any) => Transformer;
 
@@ -68,4 +68,4 @@ export type Transformer = {
     config: Config.ProjectConfig,
     options?: TransformOptions,
   ) => string | TransformedSource;
-};
+}