diff --git a/lib/config/index.js b/lib/config/index.js
index 623258da4459441bf5359f7de18b8b285beee191..37b94c60a2c2ab9e9e629d34dac4767f2e15d0e7 100644
--- a/lib/config/index.js
+++ b/lib/config/index.js
@@ -125,8 +125,9 @@ function mergeChildConfig(parentConfig, childConfig) {
     ) {
       logger.trace(`mergeable option: ${option.name}`);
       if (option.type === 'list') {
-        config[option.name] = (parentConfig[option.name] || [])
-          .concat(config[option.name] || []);
+        config[option.name] = (parentConfig[option.name] || []).concat(
+          config[option.name] || []
+        );
       } else {
         config[option.name] = {
           ...parentConfig[option.name],
diff --git a/package.json b/package.json
index d52ec8e59c9005af987dd0ece6ecc818cda502e2..3e39219df158daf5488de08c12ceb0980a574d9a 100644
--- a/package.json
+++ b/package.json
@@ -87,7 +87,7 @@
     "jest": "20.0.4",
     "mkdirp": "0.5.1",
     "mockdate": "2.0.2",
-    "prettier": "1.5.3",
+    "prettier": "1.7.0",
     "rimraf": "2.6.2",
     "semantic-release": "7.0.2"
   },
diff --git a/yarn.lock b/yarn.lock
index 6ee76d95efcd5faccbe3e31f912f66e77fda6050..a15df2f8d0f63c51da42d80fcc0fcbd41dffd891 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -4136,9 +4136,9 @@ preserve@^0.2.0:
   version "0.2.0"
   resolved "https://registry.yarnpkg.com/preserve/-/preserve-0.2.0.tgz#815ed1f6ebc65926f865b310c0713bcb3315ce4b"
 
-prettier@1.5.3:
-  version "1.5.3"
-  resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.5.3.tgz#59dadc683345ec6b88f88b94ed4ae7e1da394bfe"
+prettier@1.7.0:
+  version "1.7.0"
+  resolved "https://registry.yarnpkg.com/prettier/-/prettier-1.7.0.tgz#47481588f41f7c90f63938feb202ac82554e7150"
 
 pretty-format@^20.0.3:
   version "20.0.3"