fix: address prototype pollution issue (#108)

Author: Benjamin E. Coe

lib/index.ts

@@ -47,7 +47,7 @@ class Y18N {
     this.fallbackToLanguage = typeof opts.fallbackToLanguage === 'boolean' ? opts.fallbackToLanguage : true
 
     // internal stuff.
-    this.cache = {}
+    this.cache = Object.create(null)
     this.writeQueue = []
   }
 

test/y18n-test.cjs

@@ -351,6 +351,24 @@ describe('y18n', function () {
     })
   })
 
+  // See: https://github.com/yargs/y18n/issues/96,
+  // https://github.com/yargs/y18n/pull/107
+  describe('prototype pollution', () => {
+    it('does not pollute prototype, with __proto__ locale', () => {
+      const y = y18n()
+      y.setLocale('__proto__')
+      y.updateLocale({ polluted: '👽' })
+      y.__('polluted').should.equal('👽')
+      ;(typeof polluted).should.equal('undefined')
+    })
+
+    it('does not pollute prototype, when __ is used with __proto__ locale', () => {
+      const __ = y18n({ locale: '__proto__' }).__
+      __('hello')
+      ;(typeof {}.hello).should.equal('undefined')
+    })
+  })
+
   after(function () {
     rimraf.sync('./test/locales/fr.json')
   })