ESLint config: Permit ECMAScript 2017 syntax and globals

ECMAScript 2017 is supported by Node 10 and later.

.eslintrc.js

 module.exports = {
     "extends": "eslint:recommended",
     "env": {
+        "es2017": true,
         "node": true
     },
     "rules": {
@@ -9,4 +10,4 @@ module.exports = {
         "semi": ["error", "always"],
         "no-cond-assign": ["error", "always"]
     }
-}
+};