diff --git a/src/index.js b/src/index.js
index a604dc19..89709166 100644
--- a/src/index.js
+++ b/src/index.js
@@ -106,7 +106,7 @@ class UglifyJsPlugin {
       }
     }
 
-    return warningMessage;
+    return `｢UglifyJs｣ ${warningMessage} in ${file}`;
   }
 
   apply(compiler) {
diff --git a/test/__snapshots__/all-options.test.js.snap b/test/__snapshots__/all-options.test.js.snap
index b5c2c576..a37e679f 100644
--- a/test/__snapshots__/all-options.test.js.snap
+++ b/test/__snapshots__/all-options.test.js.snap
@@ -57,6 +57,6 @@ exports[`when applied with all options matches snapshot: manifest.d6857f782c13a9
 
 exports[`when applied with all options matches snapshot: warnings 1`] = `
 Array [
-  "Dropping unused variable a [./test/fixtures/entry.js:4,0]",
+  "｢UglifyJs｣ Dropping unused variable a [./test/fixtures/entry.js:4,0] in main.0c220ec66316af2c1b24.js",
 ]
 `;