From b8f59c884a295ed94b66e0511406ae6939c9093f Mon Sep 17 00:00:00 2001
From: Simen Bekkhus <sbekkhus91@gmail.com>
Date: Sun, 17 Feb 2019 01:18:37 +0100
Subject: [PATCH] interface over type so we can extend

---
 packages/babel-jest/src/index.ts     | 7 ++++---
 packages/jest-transform/src/types.ts | 4 ++--
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/packages/babel-jest/src/index.ts b/packages/babel-jest/src/index.ts
index cacdce09a9f9..7a23fecc252f 100644
--- a/packages/babel-jest/src/index.ts
+++ b/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 = {},
diff --git a/packages/jest-transform/src/types.ts b/packages/jest-transform/src/types.ts
index 9d3bf1dc7d6e..91df3f394de9 100644
--- a/packages/jest-transform/src/types.ts
+++ b/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;
-};
+}