diff --git a/index.js b/index.js
index 6f7634a..b8b9c3c 100644
--- a/index.js
+++ b/index.js
@@ -1,52 +1,52 @@
-const postcss = require('postcss')
+const postcss = require('postcss');
 
 module.exports = function (plugins, options, filterType) {
-  plugins = [].concat(plugins).filter(Boolean)
-  options = options || {}
+  plugins = [].concat(plugins).filter(Boolean);
+  options = options || {};
 
-  const css = postcss(plugins)
+  const css = postcss(plugins);
 
-  return function posthtmlPostcss (tree) {
-    const promises = []
+  return function (tree) {
+    const promises = [];
 
-    tree.walk(function (node) {
-      let promise
+    tree.walk(node => {
+      let promise;
 
       if (node.tag === 'style' && node.content) {
-        let meetsFilter = true
+        let meetsFilter = true;
         if (filterType) {
-          const typeAttr = (node.attrs && node.attrs.type) ? node.attrs.type.trim() : ''
-          const meetsTypeAttr = filterType.test(typeAttr)
-          const meetsStandardType = filterType.test('text/css') && (meetsTypeAttr || typeAttr === '')
-          const meetsOtherType = !meetsStandardType && meetsTypeAttr
-          meetsFilter = meetsStandardType || meetsOtherType
+          const typeAttr = (node.attrs && node.attrs.type) ? node.attrs.type.trim() : '';
+          const meetsTypeAttr = filterType.test(typeAttr);
+          const meetsStandardType = filterType.test('text/css') && (meetsTypeAttr || typeAttr === '');
+          const meetsOtherType = !meetsStandardType && meetsTypeAttr;
+          meetsFilter = meetsStandardType || meetsOtherType;
         }
 
         if (meetsFilter) {
-          const styles = [].concat(node.content).join('')
+          const styles = [].concat(node.content).join('');
           promise = css.process(styles, options)
-            .then(function (result) {
-              node.content = [result.css]
-            })
+            .then(result => {
+              node.content = [result.css];
+            });
 
-          promises.push(promise)
+          promises.push(promise);
         }
       }
 
       if (node.attrs && node.attrs.style) {
         promise = css.process(node.attrs.style, options)
-          .then(function (result) {
-            node.attrs.style = result.css
-          })
+          .then(result => {
+            node.attrs.style = result.css;
+          });
 
-        promises.push(promise)
+        promises.push(promise);
       }
 
-      return node
-    })
+      return node;
+    });
 
-    return Promise.all(promises).then(function () {
-      return tree
-    })
-  }
-}
+    return Promise.all(promises).then(() => {
+      return tree;
+    });
+  };
+};
diff --git a/test/test.js b/test/test.js
index 1af0a94..56da648 100644
--- a/test/test.js
+++ b/test/test.js
@@ -1,129 +1,133 @@
-const posthtml = require('posthtml')
-const css = require('..')
-const expect = require('chai').expect
+const posthtml = require('posthtml');
+const css = require('..');
+const {expect} = require('chai');
 
-function test (html, expected, postcssOptions, typeFilter, plugins, done) {
-  Object.assign(postcssOptions, { from: undefined })
-  plugins = plugins || [require('autoprefixer')({ overrideBrowserslist: ['ie >= 10'] })]
+function test(html, expected, postcssOptions, typeFilter, plugins, done) {
+  Object.assign(postcssOptions, {from: undefined});
+  plugins = plugins || [require('autoprefixer')({overrideBrowserslist: ['ie >= 10']})];
   expect(posthtml([css(plugins, postcssOptions, typeFilter)])
     .process(html)
-    .then(function (result) {
-      expect(expected).to.eql(result.html)
-      done()
-    }).catch(function (error) {
-      done(error)
-    }))
+    .then(result => {
+      expect(expected).to.eql(result.html);
+      done();
+    }).catch(error => {
+      done(error);
+    }));
 }
 
-describe('use postcss', function () {
-  it('object options', function () {
-    expect(function () { posthtml([css({})]) }).to.throw(Error)
-  })
-
-  it('options', function () {
-    expect(function () { posthtml([css([])]) }).to.not.throw(Error)
-  })
-
-  it('style tag', function (done) {
-    const html = '<style>a {display: flex;}</style>'
-    const expected = '<style>a {display: -ms-flexbox;display: flex;}</style>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('style tag empty', function (done) {
-    const html = '<style></style>'
-    const expected = '<style></style>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('style attrs', function (done) {
-    const html = '<div style="display: flex;"></div>'
-    const expected = '<div style="display: -ms-flexbox;display: flex;"></div>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('style attrs empty', function (done) {
-    const html = '<div style></div>'
-    const expected = '<div style=""></div>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('no style', function (done) {
-    const html = 'text <div></div>'
-    const expected = 'text <div></div>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('filtered style tag with standard type', function (done) {
-    const html = '<style type="text/css">a {display: flex;}</style>'
-    const expected = '<style type="text/css">a {display: -ms-flexbox;display: flex;}</style>'
-    test(html, expected, {}, /^text\/css$/, null, done)
-  })
-
-  it('filtered style tag with standard type (with spaces)', function (done) {
-    const html = '<style type=" text/css  ">a {display: flex;}</style>'
-    const expected = '<style type=" text/css  ">a {display: -ms-flexbox;display: flex;}</style>'
-    test(html, expected, {}, /^text\/css$/, null, done)
-  })
-
-  it('filtered style tag with standard type (empty string)', function (done) {
-    const html = '<style type="">a {display: flex;}</style>'
-    const expected = '<style type="">a {display: -ms-flexbox;display: flex;}</style>'
-    test(html, expected, {}, /^text\/css$/, null, done)
-  })
-
-  it('filtered style tag with standard type (one empty space)', function (done) {
-    const html = '<style type=" ">a {display: flex;}</style>'
-    const expected = '<style type=" ">a {display: -ms-flexbox;display: flex;}</style>'
-    test(html, expected, {}, /^text\/css$/, null, done)
-  })
-
-  it('filtered style tag with standard type (two empty spaces)', function (done) {
-    const html = '<style type="  ">a {display: flex;}</style>'
-    const expected = '<style type="  ">a {display: -ms-flexbox;display: flex;}</style>'
-    test(html, expected, {}, /^text\/css$/, null, done)
-  })
-
-  it('filtered style tag with non-standard type', function (done) {
-    const html = '<style type="text/other">a {display: flex;}</style>'
-    const expected = '<style type="text/other">a {display: -ms-flexbox;display: flex;}</style>'
-    test(html, expected, {}, /^text\/other$/, null, done)
-  })
-
-  it('filtered out style tag with non-standard type', function (done) {
-    const html = '<style type="text/other">a {display: flex;}</style>'
-    const expected = html
-    test(html, expected, {}, /^text\/another$/, null, done)
-  })
-
-  it('style tag with newline and not indent', function (done) {
-    const html = 'text <style>\n.test { color: red; }</style>'
-    const expected = 'text <style>\n.test { color: red; }</style>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('style tag with newline and multyply indent', function (done) {
-    const html = 'text <style>\n    .test {\n    color: red;\n}</style>'
-    const expected = 'text <style>\n    .test {\n    color: red;\n}</style>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('style tag with newline and indent', function (done) {
-    const html = 'text <style>\n    .test { color: red; }</style>'
-    const expected = 'text <style>\n    .test { color: red; }</style>'
-    test(html, expected, {}, null, null, done)
-  })
-
-  it('style tag with newline and indent + plugin remove "\\n" character', function (done) {
-    const html = 'text <style>\n    .test { color: red; }</style>'
-    const expected = 'text <style>    .test { color: red; }</style>'
-
-    function plugin (root) {
-      root.walk(function (node) {
-        node.raws.before = node.raws.before.replace('\n', '')
-      })
+describe('use postcss', () => {
+  it('object options', () => {
+    expect(() => {
+      posthtml([css({})]);
+    }).to.throw(Error);
+  });
+
+  it('options', () => {
+    expect(() => {
+      posthtml([css([])]);
+    }).to.not.throw(Error);
+  });
+
+  it('style tag', done => {
+    const html = '<style>a {display: flex;}</style>';
+    const expected = '<style>a {display: -ms-flexbox;display: flex;}</style>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('style tag empty', done => {
+    const html = '<style></style>';
+    const expected = '<style></style>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('style attrs', done => {
+    const html = '<div style="display: flex;"></div>';
+    const expected = '<div style="display: -ms-flexbox;display: flex;"></div>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('style attrs empty', done => {
+    const html = '<div style></div>';
+    const expected = '<div style=""></div>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('no style', done => {
+    const html = 'text <div></div>';
+    const expected = 'text <div></div>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('filtered style tag with standard type', done => {
+    const html = '<style type="text/css">a {display: flex;}</style>';
+    const expected = '<style type="text/css">a {display: -ms-flexbox;display: flex;}</style>';
+    test(html, expected, {}, /^text\/css$/, null, done);
+  });
+
+  it('filtered style tag with standard type (with spaces)', done => {
+    const html = '<style type=" text/css  ">a {display: flex;}</style>';
+    const expected = '<style type=" text/css  ">a {display: -ms-flexbox;display: flex;}</style>';
+    test(html, expected, {}, /^text\/css$/, null, done);
+  });
+
+  it('filtered style tag with standard type (empty string)', done => {
+    const html = '<style type="">a {display: flex;}</style>';
+    const expected = '<style type="">a {display: -ms-flexbox;display: flex;}</style>';
+    test(html, expected, {}, /^text\/css$/, null, done);
+  });
+
+  it('filtered style tag with standard type (one empty space)', done => {
+    const html = '<style type=" ">a {display: flex;}</style>';
+    const expected = '<style type=" ">a {display: -ms-flexbox;display: flex;}</style>';
+    test(html, expected, {}, /^text\/css$/, null, done);
+  });
+
+  it('filtered style tag with standard type (two empty spaces)', done => {
+    const html = '<style type="  ">a {display: flex;}</style>';
+    const expected = '<style type="  ">a {display: -ms-flexbox;display: flex;}</style>';
+    test(html, expected, {}, /^text\/css$/, null, done);
+  });
+
+  it('filtered style tag with non-standard type', done => {
+    const html = '<style type="text/other">a {display: flex;}</style>';
+    const expected = '<style type="text/other">a {display: -ms-flexbox;display: flex;}</style>';
+    test(html, expected, {}, /^text\/other$/, null, done);
+  });
+
+  it('filtered out style tag with non-standard type', done => {
+    const html = '<style type="text/other">a {display: flex;}</style>';
+    const expected = html;
+    test(html, expected, {}, /^text\/another$/, null, done);
+  });
+
+  it('style tag with newline and not indent', done => {
+    const html = 'text <style>\n.test { color: red; }</style>';
+    const expected = 'text <style>\n.test { color: red; }</style>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('style tag with newline and multyply indent', done => {
+    const html = 'text <style>\n    .test {\n    color: red;\n}</style>';
+    const expected = 'text <style>\n    .test {\n    color: red;\n}</style>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('style tag with newline and indent', done => {
+    const html = 'text <style>\n    .test { color: red; }</style>';
+    const expected = 'text <style>\n    .test { color: red; }</style>';
+    test(html, expected, {}, null, null, done);
+  });
+
+  it('style tag with newline and indent + plugin remove "\\n" character', done => {
+    const html = 'text <style>\n    .test { color: red; }</style>';
+    const expected = 'text <style>    .test { color: red; }</style>';
+
+    function plugin(root) {
+      root.walk(node => {
+        node.raws.before = node.raws.before.replace('\n', '');
+      });
     }
 
-    test(html, expected, {}, null, [plugin], done)
-  })
-})
+    test(html, expected, {}, null, [plugin], done);
+  });
+});
diff --git a/xo.config.js b/xo.config.js
index 25cc552..817a419 100644
--- a/xo.config.js
+++ b/xo.config.js
@@ -1,5 +1,7 @@
 module.exports = {
+  env: ['mocha'],
   space: true,
   rules: {
+    'promise/prefer-await-to-then': 'off'
   }
 };