Turn on eslint rule one-var and fix existing errors.

.eslintrc

 {
     "extends": "google",
     "plugins": ["html"],
     "rules": {
       "accessor-pairs": "off",
       "array-bracket-spacing": "off",
       "array-callback-return": "off",
       "arrow-parens": "off",
       "block-scoped-var": "off",
       "brace-style": "off",
@@ skipping 60 matching lines @@
       "no-unreachable": "error",
       "no-unused-expressions": "off",
       "no-unused-vars": "off",
       "no-use-before-define": "off",
       "no-useless-call": "off",
       "no-useless-concat": "off",
       "no-useless-escape": "off",
       "no-warning-comments": "off",
       "no-whitespace-before-property": "off",
       "object-curly-spacing": "off",
-      "one-var": "off",
+      "one-var": ["error", {
+        var: "never",
+        let: "never",
+        const: "never",
+      }],
       "one-var-declaration-per-line": "off",
       "operator-assignment": "off",
       "padded-blocks": "off",
       "quote-props": "off",
       "radix": "off",
       "require-jsdoc": "off",
       "semi": "off",
       "semi-spacing": "off",
       "space-unary-ops": "off",
       "spaced-comment": "off",
       "valid-jsdoc": "off",
       "wrap-iife": "off",
       "yoda": "off"
     }
 }