interface over type so we can extend

Author: SimenB

packages/babel-jest/src/index.ts

@@ -23,10 +23,11 @@ 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 & {
+// Narrow down the types
+interface TransformerWithFactory extends Transformer {
+  canInstrument: true;
   createTransformer: (options?: TransformOptions) => TransformerWithFactory;
-};
+}
 
 const createTransformer = (
   options: TransformOptions = {},

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;
-};
+}