diff --git a/lib/grammar-error.js b/lib/grammar-error.js
index af831d53..9f06e2b8 100644
--- a/lib/grammar-error.js
+++ b/lib/grammar-error.js
@@ -2,9 +2,19 @@
 
 // See: https://github.com/Microsoft/TypeScript-wiki/blob/master/Breaking-Changes.md#extending-built-ins-like-error-array-and-map-may-no-longer-work
 // This is roughly what typescript generates, it's not called after super(), where it's needed.
-const setProtoOf = Object.setPrototypeOf ||
-  ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
-  function (d, b) { for (var p in b) if (Object.prototype.hasOwnProperty.call(b, p)) d[p] = b[p]; };
+const setProtoOf = Object.setPrototypeOf
+  || ({ __proto__: [] } instanceof Array
+      && function(d, b) {
+        // eslint-disable-next-line no-proto
+        d.__proto__ = b;
+      })
+  || function(d, b) {
+    for (const p in b) {
+      if (Object.prototype.hasOwnProperty.call(b, p)) {
+        d[p] = b[p];
+      }
+    }
+  };
 
 // Thrown when the grammar contains an error.
 class GrammarError extends Error {