Fix undefined when no options or prefix were provided.

index.js

@@ -8,7 +8,14 @@
  *  skipGlobalSelectors?: boolean
  * }>}
  */
-const prefixPlugin = (options = {}) => {
+const prefixPlugin = options => {
+  // If options isn't set or prefix not provided, do nothing.
+  if (!options || !isObj(options) || !options.prefix) {
+    return {
+      postcssPlugin: 'postcss-prefix-selector',
+    };
+  }
+
   const prefix = options.prefix;
   const prefixWithSpace = /\s+$/.test(prefix) ? prefix : `${prefix} `;
   const ignoreFiles = options.ignoreFiles ? [].concat(options.ignoreFiles) : [];
@@ -93,6 +100,10 @@ function excludeSelector(selector, excludeArr) {
   });
 }
 
+function isObj(obj) {
+  return Object.prototype.toString.call(obj) === '[object Object]';
+}
+
 prefixPlugin.postcss = true;
 
 module.exports = prefixPlugin;

package.json

@@ -30,7 +30,7 @@
   },
   "scripts": {
     "test": "mocha",
-    "test-cov": "nyc mocha",
+    "test:cov": "nyc mocha",
     "build:types": "tsc --project tsconfig.types.json",
     "lint": "prettier --write '**/*.{js,css}'",
     "prepack": "npm run build:types",

test/test.js

@@ -4,6 +4,20 @@ const fs = require('fs');
 const prefixer = require('../index.js');
 const postcssNested = require('postcss-nested');
 
+it('should not make changes if no options are set', function () {
+  const out = postcss().use(prefixer()).process(getFixtureContents('single-selector.css')).css;
+  const expected = getFixtureContents('single-selector.css');
+
+  assert.equal(out, expected);
+});
+
+it('should not make changes if no prefix is set', function () {
+  const out = postcss().use(prefixer({})).process(getFixtureContents('single-selector.css')).css;
+  const expected = getFixtureContents('single-selector.css');
+
+  assert.equal(out, expected);
+});
+
 it('should prefix a selector', () => {
   const out = postcss()
     .use(