diff --git a/packages/jest-haste-map/src/lib/fast_path.ts b/packages/jest-haste-map/src/lib/fast_path.ts
index 255751de4a91..85a5b71d6595 100644
--- a/packages/jest-haste-map/src/lib/fast_path.ts
+++ b/packages/jest-haste-map/src/lib/fast_path.ts
@@ -9,7 +9,7 @@ import * as path from 'path';
 
 // rootDir and filename must be absolute paths (resolved)
 export function relative(rootDir: string, filename: string): string {
-  return filename.indexOf(rootDir + path.sep) === 0
+  return filename.startsWith(rootDir + path.sep)
     ? filename.slice(rootDir.length + 1)
     : path.relative(rootDir, filename);
 }
@@ -19,7 +19,7 @@ const INDIRECTION_FRAGMENT = `..${path.sep}`;
 // rootDir must be an absolute path and relativeFilename must be simple
 // (e.g.: foo/bar or ../foo/bar, but never ./foo or foo/../bar)
 export function resolve(rootDir: string, relativeFilename: string): string {
-  return relativeFilename.indexOf(INDIRECTION_FRAGMENT) === 0
+  return relativeFilename.startsWith(INDIRECTION_FRAGMENT)
     ? path.resolve(rootDir, relativeFilename)
     : rootDir + path.sep + relativeFilename;
 }