Add eslint config.

.eslintrc.cjs

@@ -0,0 +1,26 @@
+module.exports = {
+    "env": {
+        "browser": true,
+        "es2021": true
+    },
+    "extends": "eslint:recommended",
+    "overrides": [
+        {
+            "env": {
+                "node": true
+            },
+            "files": [
+                ".eslintrc.{js,cjs}"
+            ],
+            "parserOptions": {
+                "sourceType": "script"
+            }
+        }
+    ],
+    "parserOptions": {
+        "ecmaVersion": "latest",
+        "sourceType": "module"
+    },
+    "rules": {
+    }
+}

package.json

@@ -27,6 +27,7 @@
     "babel-core": "^6.18.2",
     "babel-preset-es2015": "^6.18.0",
     "chai": "*",
+    "eslint": "^8.48.0",
     "mocha": "*",
     "rollup": "^0.36.4"
   },