Chore: use optional param to simplify.

Author: aladdin-add

lib/rules/no-unused-vars.js

@@ -10,10 +10,12 @@ const utils = require('../utils')
  * Collect used variables recursively.
  *
  * @param {Node} node - The node to collect.
- * @param {Object} used - The object to restore result.
+ * @param {Object} used(optional) - The object to restore result.
  * @returns {Object} used variables.
  */
 function collectUsed (node, used) {
+  used = used || {}
+
   if (node.type === 'VExpressionContainer') {
     node.references.forEach(ref => {
       used[ref.id.name] = true
@@ -42,7 +44,7 @@ function create (context) {
   utils.registerTemplateBodyVisitor(context, {
     "VAttribute[directive=true][key.name='for']": function (node) {
       const vars = node.value.expression.left
-      const used = collectUsed(node.parent.parent, {})
+      const used = collectUsed(node.parent.parent)
 
       // report unused.
       vars.filter(v => !used[v.name]).forEach(v => {
@@ -58,7 +60,7 @@ function create (context) {
     },
 
     "VAttribute[directive=false][key.name='scope']": function (node) {
-      const used = collectUsed(node.parent.parent, {})
+      const used = collectUsed(node.parent.parent)
 
       if (!used[node.value.value]) {
         context.report({