diff --git a/index.js b/index.js
index 538c1db..0f45e6d 100644
--- a/index.js
+++ b/index.js
@@ -6,12 +6,18 @@ const defaults = {
   rootValue: 16,
   unitPrecision: 5,
   selectorBlackList: [],
-  propList: ["font", "font-size", "line-height", "letter-spacing", "word-spacing"],
+  propList: [
+    "font",
+    "font-size",
+    "line-height",
+    "letter-spacing",
+    "word-spacing"
+  ],
   replace: true,
   mediaQuery: false,
   minPixelValue: 0,
   exclude: null,
-  unit: "px",
+  unit: "px"
 };
 
 const legacyOptions = {
@@ -20,7 +26,7 @@ const legacyOptions = {
   selector_black_list: "selectorBlackList",
   prop_white_list: "propList",
   media_query: "mediaQuery",
-  propWhiteList: "propList",
+  propWhiteList: "propList"
 };
 
 function convertLegacyOptions(options) {
@@ -36,7 +42,7 @@ function convertLegacyOptions(options) {
     delete options["prop_white_list"];
     delete options.propWhiteList;
   }
-  Object.keys(legacyOptions).forEach((key) => {
+  Object.keys(legacyOptions).forEach(key => {
     if (Reflect.has(options, key)) {
       options[legacyOptions[key]] = options[key];
       delete options[key];
@@ -61,12 +67,12 @@ function toFixed(number, precision) {
 }
 
 function declarationExists(decls, prop, value) {
-  return decls.some((decl) => decl.prop === prop && decl.value === value);
+  return decls.some(decl => decl.prop === prop && decl.value === value);
 }
 
 function blacklistedSelector(blacklist, selector) {
   if (typeof selector !== "string") return;
-  return blacklist.some((regex) => {
+  return blacklist.some(regex => {
     if (typeof regex === "string") {
       return selector.indexOf(regex) !== -1;
     }
@@ -85,31 +91,31 @@ function createPropListMatcher(propList) {
     notExact: filterPropList.notExact(propList),
     notContain: filterPropList.notContain(propList),
     notStartWith: filterPropList.notStartWith(propList),
-    notEndWith: filterPropList.notEndWith(propList),
+    notEndWith: filterPropList.notEndWith(propList)
   };
-  return (prop) => {
+  return prop => {
     if (matchAll) return true;
     return (
       (hasWild ||
         lists.exact.indexOf(prop) > -1 ||
-        lists.contain.some(function (m) {
+        lists.contain.some(function(m) {
           return prop.indexOf(m) > -1;
         }) ||
-        lists.startWith.some(function (m) {
+        lists.startWith.some(function(m) {
           return prop.indexOf(m) === 0;
         }) ||
-        lists.endWith.some(function (m) {
+        lists.endWith.some(function(m) {
           return prop.indexOf(m) === prop.length - m.length;
         })) &&
       !(
         lists.notExact.indexOf(prop) > -1 ||
-        lists.notContain.some(function (m) {
+        lists.notContain.some(function(m) {
           return prop.indexOf(m) > -1;
         }) ||
-        lists.notStartWith.some(function (m) {
+        lists.notStartWith.some(function(m) {
           return prop.indexOf(m) === 0;
         }) ||
-        lists.notEndWith.some(function (m) {
+        lists.notEndWith.some(function(m) {
           return prop.indexOf(m) === prop.length - m.length;
         })
       )
@@ -122,62 +128,69 @@ module.exports = (options = {}) => {
   const opts = Object.assign({}, defaults, options);
   const satisfyPropList = createPropListMatcher(opts.propList);
   const exclude = opts.exclude;
-  let isExcludeFile = false;
-  let pxReplace;
   return {
     postcssPlugin: "postcss-pxtorem",
-    Once(css) {
-      const filePath = css.source.input.file;
-      if (
-        exclude &&
-        ((type.isFunction(exclude) && exclude(filePath)) ||
-          (type.isString(exclude) && filePath.indexOf(exclude) !== -1) ||
-          filePath.match(exclude) !== null)
-      ) {
-        isExcludeFile = true;
-      } else {
-        isExcludeFile = false;
-      }
+    prepare() {
+      let isExcludeFile = false;
+      let pxReplace;
+      return {
+        Once(css) {
+          const filePath = css.source.input.file;
+          if (
+            exclude &&
+            ((type.isFunction(exclude) && exclude(filePath)) ||
+              (type.isString(exclude) && filePath.indexOf(exclude) !== -1) ||
+              filePath.match(exclude) !== null)
+          ) {
+            isExcludeFile = true;
+          } else {
+            isExcludeFile = false;
+          }
 
-      const rootValue =
-        typeof opts.rootValue === "function"
-          ? opts.rootValue(css.source.input)
-          : opts.rootValue;
-      pxReplace = createPxReplace(
-        rootValue,
-        opts.unitPrecision,
-        opts.minPixelValue,
-      );
-    },
-    Declaration(decl) {
-      if (isExcludeFile) return;
+          const rootValue =
+            typeof opts.rootValue === "function"
+              ? opts.rootValue(css.source.input)
+              : opts.rootValue;
+          pxReplace = createPxReplace(
+            rootValue,
+            opts.unitPrecision,
+            opts.minPixelValue
+          );
+        },
+        Declaration(decl) {
+          if (isExcludeFile) return;
 
-      if (
-        decl.value.indexOf(opts.unit) === -1 ||
-        !satisfyPropList(decl.prop) ||
-        blacklistedSelector(opts.selectorBlackList, decl.parent.selector)
-      )
-        return;
+          if (
+            decl.value.indexOf(opts.unit) === -1 ||
+            !satisfyPropList(decl.prop) ||
+            blacklistedSelector(opts.selectorBlackList, decl.parent.selector)
+          )
+            return;
 
-      const value = decl.value.replace(pxRegex(opts.unit), pxReplace);
+          const value = decl.value.replace(pxRegex(opts.unit), pxReplace);
 
-      // if rem unit already exists, do not add or replace
-      if (declarationExists(decl.parent, decl.prop, value)) return;
+          // if rem unit already exists, do not add or replace
+          if (declarationExists(decl.parent, decl.prop, value)) return;
 
-      if (opts.replace) {
-        decl.value = value;
-      } else {
-        decl.cloneAfter({ value: value });
-      }
-    },
-    AtRule(atRule) {
-      if (isExcludeFile) return;
+          if (opts.replace) {
+            decl.value = value;
+          } else {
+            decl.cloneAfter({ value: value });
+          }
+        },
+        AtRule(atRule) {
+          if (isExcludeFile) return;
 
-      if (opts.mediaQuery && atRule.name === "media") {
-        if (atRule.params.indexOf(opts.unit) === -1) return;
-        atRule.params = atRule.params.replace(pxRegex(opts.unit), pxReplace);
-      }
-    },
+          if (opts.mediaQuery && atRule.name === "media") {
+            if (atRule.params.indexOf(opts.unit) === -1) return;
+            atRule.params = atRule.params.replace(
+              pxRegex(opts.unit),
+              pxReplace
+            );
+          }
+        }
+      };
+    }
   };
 };
 module.exports.postcss = true;