diff --git a/src/ExistingJsProject.res b/src/ExistingJsProject.res
index 149f7e1..fb94207 100644
--- a/src/ExistingJsProject.res
+++ b/src/ExistingJsProject.res
@@ -38,12 +38,6 @@ let updateRescriptJson = async (~projectName, ~sourceDir, ~moduleSystem, ~suffix
     }
   )
 
-let getSuffixForModuleSystem = moduleSystem =>
-  switch moduleSystem {
-  | "esmodule" | "es6" | "es6-global" => ".res.mjs"
-  | _ => ".res.js"
-  }
-
 let getModuleSystemOptions = (~versions) => [
   {
     P.value: "commonjs",
@@ -72,7 +66,7 @@ let addToExistingProject = async (~projectName) => {
     options: getModuleSystemOptions(~versions),
   })->P.resultOrRaise
 
-  let suffix = moduleSystem->getSuffixForModuleSystem
+  let suffix = moduleSystem->ModuleSystem.getSuffix
 
   let shouldCheckJsFilesIntoGit = await P.confirm({
     message: `Do you want to check generated ${suffix} files into git?`,
diff --git a/src/ModuleSystem.res b/src/ModuleSystem.res
new file mode 100644
index 0000000..92be20b
--- /dev/null
+++ b/src/ModuleSystem.res
@@ -0,0 +1,5 @@
+let getSuffix = moduleSystem =>
+  switch moduleSystem {
+  | "esmodule" | "es6" | "es6-global" => ".res.mjs"
+  | _ => ".res.js"
+  }
diff --git a/src/NewProject.res b/src/NewProject.res
index 20ab6a3..e977a5e 100644
--- a/src/NewProject.res
+++ b/src/NewProject.res
@@ -32,6 +32,9 @@ let updateRescriptJson = async (~projectName, ~versions) =>
       | Some(Object(packageSpecs)) | Some(Array([Object(packageSpecs)])) =>
         let moduleSystemName = versions->RescriptVersions.esmModuleSystemName
         packageSpecs->Dict.set("module", String(moduleSystemName))
+
+        let suffix = moduleSystemName->ModuleSystem.getSuffix
+        config->Dict.set("suffix", String(suffix))
       | _ => ()
       }
     | _ => ()