diff --git a/packages/jest-transform/src/ScriptTransformer.ts b/packages/jest-transform/src/ScriptTransformer.ts
index 1864319361c2..b1ab7a935f45 100644
--- a/packages/jest-transform/src/ScriptTransformer.ts
+++ b/packages/jest-transform/src/ScriptTransformer.ts
@@ -759,6 +759,7 @@ class ScriptTransformer {
         }
       },
       {
+        // Exclude `mjs` extension when addHook because pirates don't support hijack es module
         exts: this._config.moduleFileExtensions
           .filter(ext => ext !== 'mjs')
           .map(ext => `.${ext}`),