diff --git a/.eslintrc.yml b/.eslintrc.yml
index 65f1dce6c..73eeec276 100644
--- a/.eslintrc.yml
+++ b/.eslintrc.yml
@@ -12,3 +12,4 @@ rules:
   eqeqeq: error
   no-warning-comments: warn
   no-var: error
+  prefer-const: error
diff --git a/samples/iam.js b/samples/iam.js
index ab7ca726d..703bc6604 100644
--- a/samples/iam.js
+++ b/samples/iam.js
@@ -132,7 +132,7 @@ function removeBucketIamMember(bucketName, roleName, members) {
 
       // Finds and updates the appropriate role-member group
       const index = policy.bindings.findIndex(role => role.role === roleName);
-      let role = policy.bindings[index];
+      const role = policy.bindings[index];
       if (role) {
         role.members = role.members.filter(
           member => members.indexOf(member) === -1
diff --git a/samples/system-test/encryption.test.js b/samples/system-test/encryption.test.js
index 00af29f18..798f9e792 100644
--- a/samples/system-test/encryption.test.js
+++ b/samples/system-test/encryption.test.js
@@ -108,7 +108,7 @@ test.serial(`should rotate keys`, async t => {
   const output = generateKeyResults.stdout + generateKeyResults.stderr;
   t.regex(output, new RegExp(`Base 64 encoded encryption key:`));
   const test = /^Base 64 encoded encryption key: (.+)$/;
-  let newKey = output.match(test)[1];
+  const newKey = output.match(test)[1];
 
   const results = await tools.runAsyncWithIO(
     `${cmd} rotate ${bucketName} ${fileName} ${key} ${newKey}`,