Tweak ESLint rules.

js/.eslintrc.json

@@ -14,9 +14,8 @@
   "rules": {
     // Possible Errors
     "no-await-in-loop": "error",
-    "no-compare-neg-zero": "error",
     "no-extra-parens": "error",
-    "no-prototype-builtins": "off",
+    "no-prototype-builtins": "error",
     "no-template-curly-in-string": "error",
     "valid-jsdoc": "error",
 
@@ -85,7 +84,6 @@
     "no-unused-expressions": "error",
     "no-useless-call": "error",
     "no-useless-concat": "error",
-    "no-useless-escape": "error",
     "no-useless-return": "off",
     "no-void": "error",
     "no-warning-comments": "off",
@@ -108,7 +106,7 @@
     "no-shadow": "off",
     "no-shadow-restricted-names": "error",
     "no-undef-init": "error",
-    "no-undefined": "off",
+    "no-undefined": "error",
     "no-use-before-define": "off",
 
     // Node.js and CommonJS
@@ -213,7 +211,7 @@
     "wrap-regex": "off",
 
     // ECMAScript 6
-    "arrow-body-style": "off",
+    "arrow-body-style": ["error", "as-needed"],
     "arrow-parens": "error",
     "arrow-spacing": "error",
     "generator-star-spacing": "error",