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

.eslintrc

Index: .eslintrc
diff --git a/.eslintrc b/.eslintrc
index 2882258a48b36e6e994caa7d4258311a3967f6e2..428c7b91e19eaef6852215b064b2282c42c11fb5 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -78,7 +78,11 @@
       "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",