refactor: migrate to ES6

Author: SukkaW

lib/api.js

@@ -80,14 +80,14 @@ function walk (cb) {
  */
 function match (expression, cb) {
   return Array.isArray(expression)
-    ? traverse(this, function (node) {
-      for (var i = 0; i < expression.length; i++) {
+    ? traverse(this, node => {
+      for (let i = 0; i < expression.length; i++) {
         if (compare(expression[i], node)) return cb(node)
       }
 
       return node
     })
-    : traverse(this, function (node) {
+    : traverse(this, node => {
       if (compare(expression, node)) return cb(node)
 
       return node
@@ -101,7 +101,7 @@ module.exports.walk = walk
 /** @private */
 function traverse (tree, cb) {
   if (Array.isArray(tree)) {
-    for (var i = 0; i < tree.length; i++) {
+    for (let i = 0; i < tree.length; i++) {
       tree[i] = traverse(cb(tree[i]), cb)
     }
   } else if (
@@ -126,16 +126,12 @@ function compare (expected, actual) {
   }
 
   if (Array.isArray(expected)) {
-    return expected.every(function (exp) {
-      return [].some.call(actual, function (act) {
-        return compare(exp, act)
-      })
-    })
+    return expected.every(exp => [].some.call(actual, act => compare(exp, act)))
   }
 
-  return Object.keys(expected).every(function (key) {
-    var ao = actual[key]
-    var eo = expected[key]
+  return Object.keys(expected).every(key => {
+    const ao = actual[key]
+    const eo = expected[key]
 
     if (typeof eo === 'object' && eo !== null && ao !== null) {
       return compare(eo, ao)