docs: modules

docs/en/modules.md

@@ -1 +1,89 @@
 # Modules
+
+Due to using a single state tree, all state of our application is contained inside one big object. However as our application grows in scale, the store can get really bloated.
+
+To help with that, Vuex allows us to divide our store into **modules**. Each module can contain its own state, mutations, actions, getters, and even nested modules - it's fractal all the way down:
+
+``` js
+const moduleA = {
+  state: { ... },
+  mutations: { ... },
+  actions: { ... }
+}
+
+const moduleB = {
+  state: { ... },
+  mutations: { ... },
+  actions: { ... }
+}
+
+const store = new Vuex.Store({
+  modules: {
+    a: moduleA,
+    b: moduleB
+  }
+})
+
+store.state.a // -> moduleA's state
+store.state.b // -> moduleB's state
+```
+
+### Module Local State
+
+Inside a module's mutations and getters, The first argument received will be **the module's local state** instead of root state:
+
+``` js
+cosnt moduleA = {
+  state: { count: 0 },
+  mutations: {
+    increment: (state, rootState) {
+      // state is the local module state
+      state.count++
+
+      // rootState is passed as the second argument, but you should not
+      // mutate it from within a module.
+    }
+  },
+
+  getters: {
+    doubleCount (state) {
+      return state.count * 2
+    }
+  }
+}
+```
+
+Similarly, inside module actions, `context.state` will expose the local state, and root state will be exposed as `context.rootState`:
+
+``` js
+const moduleA = {
+  // ...
+  actions: {
+    incrementIfOdd ({ state, commit }) {
+      if (state.count % 2 === 1) {
+        commit('increment')
+      }
+    }
+  }
+}
+```
+
+### Namespacing
+
+Note that actions, mutations and getters inside modules are still registered under the **global namespace** - this allows multiple modules to react to the same mutation/action type. You can namespace the module assets yourself to avoid name clashing, and you probably should if you are writing a reusable Vuex module that will be used in unknown environments.
+
+### Dynamic Module Registration
+
+You can register a module **after** the store has been created with the `store.reigsterModule` method:
+
+``` js
+store.registerModule('myModule', {
+  // ...
+})
+```
+
+The module's state will be exposed as `store.state.myModule`.
+
+Dynamic module registration makes it possible for other Vue plugins to also leverage Vuex for state management by attaching a module to the application's store. For example, the [`vuex-router-sync`](https://github.com/vuejs/vuex-router-sync) library integrates vue-router with vuex by managing the application's route state in a dynamically attached module.
+
+You can also remove a dynamically registered module with `store.unregisterModule(moduleName)`. Note you cannot remove static modules (declared at store creation) with this method.