diff --git a/packages/xo-collection/README.md b/packages/xo-collection/README.md
index 3659b46d9..327af3662 100644
--- a/packages/xo-collection/README.md
+++ b/packages/xo-collection/README.md
@@ -198,15 +198,15 @@ col.on('finish', () => {
 ### Iteration
 
 ```javascript
-for (let [key, value] of col) {
+for (const [key, value] of col) {
   console.log('- %s: %j', key, value)
 }
 
-for (let key of col.keys()) {
+for (const key of col.keys()) {
   console.log('- %s', key)
 }
 
-for (let value of col.values()) {
+for (const value of col.values()) {
   console.log('- %j', value)
 }
 ```
diff --git a/packages/xo-collection/src/clear-object.js b/packages/xo-collection/src/clear-object.js
index 4e5882ac6..0250f65c2 100644
--- a/packages/xo-collection/src/clear-object.js
+++ b/packages/xo-collection/src/clear-object.js
@@ -1,5 +1,5 @@
 export default function clearObject (object) {
-  for (let key in object) {
+  for (const key in object) {
     delete object[key]
   }
 }
diff --git a/packages/xo-collection/src/collection.js b/packages/xo-collection/src/collection.js
index 6b312f6a6..72a72d78e 100644
--- a/packages/xo-collection/src/collection.js
+++ b/packages/xo-collection/src/collection.js
@@ -219,7 +219,7 @@ export default class Collection extends EventEmitter {
   * [Symbol.iterator] () {
     const {_items: items} = this
 
-    for (let key in items) {
+    for (const key in items) {
       yield [key, items[key]]
     }
   }
@@ -227,7 +227,7 @@ export default class Collection extends EventEmitter {
   * keys () {
     const {_items: items} = this
 
-    for (let key in items) {
+    for (const key in items) {
       yield key
     }
   }
@@ -235,7 +235,7 @@ export default class Collection extends EventEmitter {
   * values () {
     const {_items: items} = this
 
-    for (let key in items) {
+    for (const key in items) {
       yield items[key]
     }
   }
@@ -271,7 +271,7 @@ export default class Collection extends EventEmitter {
         update: Object.create(null)
       }
 
-      for (let key in this._buffer) {
+      for (const key in this._buffer) {
         data[buffer[key]][key] = this._items[key]
       }
 
diff --git a/packages/xo-collection/src/index.js b/packages/xo-collection/src/index.js
index 76c1af5d7..78535713c 100644
--- a/packages/xo-collection/src/index.js
+++ b/packages/xo-collection/src/index.js
@@ -31,7 +31,7 @@ export default class Index {
   // Remove empty items lists.
   sweep () {
     const {_itemsByHash: itemsByHash} = this
-    for (let hash in itemsByHash) {
+    for (const hash in itemsByHash) {
       if (isEmpty(itemsByHash[hash])) {
         delete itemsByHash[hash]
       }
@@ -76,7 +76,7 @@ export default class Index {
       _keysToHash: keysToHash
     } = this
 
-    for (let key in items) {
+    for (const key in items) {
       const value = items[key]
 
       const hash = computeHash(value, key)
@@ -103,7 +103,7 @@ export default class Index {
       _keysToHash: keysToHash
     } = this
 
-    for (let key in items) {
+    for (const key in items) {
       const value = items[key]
 
       const prev = keysToHash[key]
@@ -134,7 +134,7 @@ export default class Index {
       _keysToHash: keysToHash
     } = this
 
-    for (let key in items) {
+    for (const key in items) {
       const prev = keysToHash[key]
       if (prev != null) {
         delete itemsByHash[prev][key]
diff --git a/packages/xo-collection/src/is-empty.js b/packages/xo-collection/src/is-empty.js
index a3ad65cb1..9e688ac7c 100644
--- a/packages/xo-collection/src/is-empty.js
+++ b/packages/xo-collection/src/is-empty.js
@@ -1,6 +1,6 @@
 export default function isEmpty (object) {
   /* eslint no-unused-vars: 0 */
-  for (let key in object) {
+  for (const key in object) {
     return false
   }
   return true
diff --git a/packages/xo-collection/src/unique-index.js b/packages/xo-collection/src/unique-index.js
index 6153bbe61..8a1d91b70 100644
--- a/packages/xo-collection/src/unique-index.js
+++ b/packages/xo-collection/src/unique-index.js
@@ -65,7 +65,7 @@ export default class UniqueIndex {
       _keysToHash: keysToHash
     } = this
 
-    for (let key in items) {
+    for (const key in items) {
       const value = items[key]
 
       const hash = computeHash(value, key)
@@ -84,7 +84,7 @@ export default class UniqueIndex {
       _keysToHash: keysToHash
     } = this
 
-    for (let key in items) {
+    for (const key in items) {
       const value = items[key]
 
       const prev = keysToHash[key]
@@ -109,7 +109,7 @@ export default class UniqueIndex {
       _keysToHash: keysToHash
     } = this
 
-    for (let key in items) {
+    for (const key in items) {
       const prev = keysToHash[key]
       if (prev != null) {
         delete itemByHash[prev]