Cache extension settings using has.

package.json

@@ -77,6 +77,7 @@
     "es6-map": "^0.1.3",
     "es6-set": "^0.1.4",
     "eslint-import-resolver-node": "^0.2.0",
+    "has": "^1.0.1",
     "lodash.cond": "^4.3.0",
     "lodash.endswith": "^4.0.1",
     "lodash.find": "^4.3.0",

resolvers/webpack/index.js

@@ -10,6 +10,7 @@ var findRoot = require('find-root')
   , assign = require('object-assign')
   , resolve = require('resolve')
   , semver = require('semver')
+  , has = require('has')
 
 var log = require('debug')('eslint-plugin-import:resolver:webpack')
 
@@ -265,7 +266,7 @@ function findExternal(source, externals, context) {
 
   // else, vanilla object
   for (var key in externals) {
-    if (!externals.hasOwnProperty(key)) continue
+    if (!has(externals, key)) continue
     if (source === key) return true
   }
   return false

src/rules/extensions.js

@@ -1,20 +1,27 @@
 import path from 'path'
 import endsWith from 'lodash.endswith'
+import has from 'has'
+import assign from 'object-assign'
 
 import resolve from '../core/resolve'
 import { isBuiltIn } from '../core/importType'
 
 module.exports = function (context) {
   const configuration = context.options[0] || 'never'
   const defaultConfig = typeof configuration === 'string' ? configuration : null
-  const modifiers = typeof configuration === 'object' ? configuration : context.options[1] || {}
+  const modifiers = assign(
+    {},
+    typeof configuration === 'object' ? configuration : context.options[1]
+  )
 
   function isUseOfExtensionRequired(extension) {
-    return (modifiers[extension] || defaultConfig) === 'always'
+    if (!has(modifiers, extension)) { modifiers[extension] = defaultConfig }
+    return modifiers[extension] === 'always'
   }
 
   function isUseOfExtensionForbidden(extension) {
-    return (modifiers[extension] || defaultConfig) === 'never'
+    if (!has(modifiers, extension)) { modifiers[extension] = defaultConfig }
+    return modifiers[extension] === 'never'
   }
 
   function isResolvableWithoutExtension(file) {