diff --git a/packages/xo-collection/src/collection.js b/packages/xo-collection/src/collection.js
index 8d9c74480..9875b4ff5 100644
--- a/packages/xo-collection/src/collection.js
+++ b/packages/xo-collection/src/collection.js
@@ -13,6 +13,10 @@ const {
   prototype: { hasOwnProperty }
 } = Object
 
+export const ACTION_ADD = 'add'
+export const ACTION_UPDATE = 'update'
+export const ACTION_REMOVE = 'remove'
+
 // ===================================================================
 
 export class BufferAlreadyFlushed extends BaseError {
@@ -105,14 +109,14 @@ export default class Collection extends EventEmitter {
 
     this._items[key] = value
     this._size++
-    this._touch('add', key)
+    this._touch(ACTION_ADD, key)
   }
 
   clear () {
     forEach(this._items, (_, key) => {
       delete this._items[key]
       this._size--
-      this._touch('remove', key)
+      this._touch(ACTION_REMOVE, key)
     })
   }
 
@@ -122,15 +126,15 @@ export default class Collection extends EventEmitter {
 
     delete this._items[key]
     this._size--
-    this._touch('remove', key)
+    this._touch(ACTION_REMOVE, key)
   }
 
   set (keyOrObjectWithId, valueIfKey = undefined) {
     const [key, value] = this._resolveItem(keyOrObjectWithId, valueIfKey)
 
-    const action = this.has(key) ? 'update' : 'add'
+    const action = this.has(key) ? ACTION_UPDATE : ACTION_ADD
     this._items[key] = value
-    if (action === 'add') {
+    if (action === ACTION_ADD) {
       this._size++
     }
     this._touch(action, key)
@@ -144,7 +148,7 @@ export default class Collection extends EventEmitter {
       throw new IllegalTouch(value)
     }
 
-    this._touch('update', key)
+    this._touch(ACTION_UPDATE, key)
 
     return this.get(key)
   }
@@ -155,7 +159,7 @@ export default class Collection extends EventEmitter {
     if (this.has(key)) {
       delete this._items[key]
       this._size--
-      this._touch('remove', key)
+      this._touch(ACTION_REMOVE, key)
     }
   }
 
@@ -164,7 +168,7 @@ export default class Collection extends EventEmitter {
     this._assertHas(key)
 
     this._items[key] = value
-    this._touch('update', key)
+    this._touch(ACTION_UPDATE, key)
   }
 
   // -----------------------------------------------------------------
@@ -344,17 +348,17 @@ export default class Collection extends EventEmitter {
       process.nextTick(flush)
     }
 
-    if (action === 'add') {
-      this._buffer[key] = this._buffer[key] ? 'update' : 'add'
-    } else if (action === 'remove') {
-      if (this._buffer[key] === 'add') {
+    if (action === ACTION_ADD) {
+      this._buffer[key] = this._buffer[key] ? ACTION_UPDATE : ACTION_ADD
+    } else if (action === ACTION_REMOVE) {
+      if (this._buffer[key] === ACTION_ADD) {
         delete this._buffer[key]
       } else {
-        this._buffer[key] = 'remove'
+        this._buffer[key] = ACTION_REMOVE
       }
     } else { // update
       if (!this._buffer[key]) {
-        this._buffer[key] = 'update'
+        this._buffer[key] = ACTION_UPDATE
       }
     }
   }
diff --git a/packages/xo-collection/src/index.js b/packages/xo-collection/src/index.js
index 78535713c..d3ea4d8b8 100644
--- a/packages/xo-collection/src/index.js
+++ b/packages/xo-collection/src/index.js
@@ -4,6 +4,11 @@ import callback from 'lodash.callback'
 import clearObject from './clear-object'
 import isEmpty from './is-empty'
 import NotImplemented from './not-implemented'
+import {
+  ACTION_ADD,
+  ACTION_UPDATE,
+  ACTION_REMOVE
+} from './collection'
 
 // ===================================================================
 
@@ -53,15 +58,15 @@ export default class Index {
     // has not been emitted yet.
     this._onAdd(collection.all)
 
-    collection.on('add', this._onAdd)
-    collection.on('update', this._onUpdate)
-    collection.on('remove', this._onRemove)
+    collection.on(ACTION_ADD, this._onAdd)
+    collection.on(ACTION_UPDATE, this._onUpdate)
+    collection.on(ACTION_REMOVE, this._onRemove)
   }
 
   _detachCollection (collection) {
-    collection.removeListener('add', this._onAdd)
-    collection.removeListener('update', this._onUpdate)
-    collection.removeListener('remove', this._onRemove)
+    collection.removeListener(ACTION_ADD, this._onAdd)
+    collection.removeListener(ACTION_UPDATE, this._onUpdate)
+    collection.removeListener(ACTION_REMOVE, this._onRemove)
 
     clearObject(this._itemsByHash)
     clearObject(this._keysToHash)
diff --git a/packages/xo-collection/src/unique-index.js b/packages/xo-collection/src/unique-index.js
index 8a1d91b70..815e11990 100644
--- a/packages/xo-collection/src/unique-index.js
+++ b/packages/xo-collection/src/unique-index.js
@@ -3,6 +3,11 @@ import callback from 'lodash.callback'
 
 import clearObject from './clear-object'
 import NotImplemented from './not-implemented'
+import {
+  ACTION_ADD,
+  ACTION_UPDATE,
+  ACTION_REMOVE
+} from './collection'
 
 // ===================================================================
 
@@ -42,15 +47,15 @@ export default class UniqueIndex {
     // has not been emitted yet.
     this._onAdd(collection.all)
 
-    collection.on('add', this._onAdd)
-    collection.on('update', this._onUpdate)
-    collection.on('remove', this._onRemove)
+    collection.on(ACTION_ADD, this._onAdd)
+    collection.on(ACTION_UPDATE, this._onUpdate)
+    collection.on(ACTION_REMOVE, this._onRemove)
   }
 
   _detachCollection (collection) {
-    collection.removeListener('add', this._onAdd)
-    collection.removeListener('update', this._onUpdate)
-    collection.removeListener('remove', this._onRemove)
+    collection.removeListener(ACTION_ADD, this._onAdd)
+    collection.removeListener(ACTION_UPDATE, this._onUpdate)
+    collection.removeListener(ACTION_REMOVE, this._onRemove)
 
     clearObject(this._itemByHash)
     clearObject(this._keysToHash)
diff --git a/packages/xo-collection/src/view.js b/packages/xo-collection/src/view.js
index b7c318b5c..be0b67871 100644
--- a/packages/xo-collection/src/view.js
+++ b/packages/xo-collection/src/view.js
@@ -2,7 +2,11 @@ import bind from 'lodash.bind'
 import createCallback from 'lodash.callback'
 import forEach from 'lodash.foreach'
 
-import Collection from './collection'
+import Collection, {
+  ACTION_ADD,
+  ACTION_UPDATE,
+  ACTION_REMOVE
+} from './collection'
 
 // ===================================================================
 
@@ -22,17 +26,17 @@ export default class View extends Collection {
     this._onRemove = bind(this._onRemove, this)
 
     // Register listeners.
-    this._collection.on('add', this._onAdd)
-    this._collection.on('update', this._onUpdate)
-    this._collection.on('remove', this._onRemove)
+    this._collection.on(ACTION_ADD, this._onAdd)
+    this._collection.on(ACTION_UPDATE, this._onUpdate)
+    this._collection.on(ACTION_REMOVE, this._onRemove)
   }
 
   // This method is necessary to free the memory of the view if its
   // life span is shorter than the collection.
   destroy () {
-    this._collection.removeListener('add', this._onAdd)
-    this._collection.removeListener('update', this._onUpdate)
-    this._collection.removeListener('remove', this._onRemove)
+    this._collection.removeListener(ACTION_ADD, this._onAdd)
+    this._collection.removeListener(ACTION_UPDATE, this._onUpdate)
+    this._collection.removeListener(ACTION_REMOVE, this._onRemove)
   }
 
   add () {