diff --git a/packages/google-cloud-language/.eslintrc.yml b/packages/google-cloud-language/.eslintrc.yml
index 65f1dce6c0c..73eeec27612 100644
--- a/packages/google-cloud-language/.eslintrc.yml
+++ b/packages/google-cloud-language/.eslintrc.yml
@@ -12,3 +12,4 @@ rules:
   eqeqeq: error
   no-warning-comments: warn
   no-var: error
+  prefer-const: error
diff --git a/packages/google-cloud-language/smoke-test/language_service_smoke_test.js b/packages/google-cloud-language/smoke-test/language_service_smoke_test.js
index 8acc7ed10ff..2a3d5f7e2dd 100644
--- a/packages/google-cloud-language/smoke-test/language_service_smoke_test.js
+++ b/packages/google-cloud-language/smoke-test/language_service_smoke_test.js
@@ -28,7 +28,8 @@ describe('LanguageServiceSmokeTest', () => {
       content: content,
       type: type,
     };
-    client.analyzeSentiment({document: document})
+    client
+      .analyzeSentiment({document: document})
       .then(responses => {
         const response = responses[0];
         console.log(response);
diff --git a/packages/google-cloud-language/src/v1/language_service_client.js b/packages/google-cloud-language/src/v1/language_service_client.js
index 74309f84520..bb988b69079 100644
--- a/packages/google-cloud-language/src/v1/language_service_client.js
+++ b/packages/google-cloud-language/src/v1/language_service_client.js
@@ -127,7 +127,7 @@ class LanguageServiceClient {
       'classifyText',
       'annotateText',
     ];
-    for (let methodName of languageServiceStubMethods) {
+    for (const methodName of languageServiceStubMethods) {
       this._innerApiCalls[methodName] = gax.createApiCall(
         languageServiceStub.then(
           stub =>
diff --git a/packages/google-cloud-language/src/v1beta2/language_service_client.js b/packages/google-cloud-language/src/v1beta2/language_service_client.js
index 75007fe8782..8f15b038512 100644
--- a/packages/google-cloud-language/src/v1beta2/language_service_client.js
+++ b/packages/google-cloud-language/src/v1beta2/language_service_client.js
@@ -127,7 +127,7 @@ class LanguageServiceClient {
       'classifyText',
       'annotateText',
     ];
-    for (let methodName of languageServiceStubMethods) {
+    for (const methodName of languageServiceStubMethods) {
       this._innerApiCalls[methodName] = gax.createApiCall(
         languageServiceStub.then(
           stub =>
diff --git a/packages/google-cloud-language/system-test/language_service_smoke_test.js b/packages/google-cloud-language/system-test/language_service_smoke_test.js
index ab60d5b6ab1..7d34a4b3606 100644
--- a/packages/google-cloud-language/system-test/language_service_smoke_test.js
+++ b/packages/google-cloud-language/system-test/language_service_smoke_test.js
@@ -18,20 +18,20 @@ describe('LanguageServiceSmokeTest', () => {
   it('successfully makes a call to the service', done => {
     const language = require('../src');
 
-    let client = new language.v1.LanguageServiceClient({
+    const client = new language.v1.LanguageServiceClient({
       // optional auth parameters.
     });
 
-    let content = 'Hello, world!';
-    let type = 'PLAIN_TEXT';
-    let document = {
+    const content = 'Hello, world!';
+    const type = 'PLAIN_TEXT';
+    const document = {
       content: content,
       type: type,
     };
     client
       .analyzeSentiment({document: document})
       .then(responses => {
-        let response = responses[0];
+        const response = responses[0];
         console.log(response);
       })
       .then(done)