Replace unnecessary lets with consts (#2931)

Author: Oblosys

docs/recipes/ReducingBoilerplate.md

@@ -133,7 +133,7 @@ You can always write a function that generates an action creator:
 ```js
 function makeActionCreator(type, ...argNames) {
   return function (...args) {
-    let action = { type }
+    const action = { type }
     argNames.forEach((arg, index) => {
       action[argNames[index]] = args[index]
     })
@@ -198,7 +198,7 @@ import {
 class Posts extends Component {
   loadData(userId) {
     // Injected into props by React Redux `connect()` call:
-    let { dispatch, posts } = this.props
+    const { dispatch, posts } = this.props
 
     if (posts[userId]) {
       // There is cached data! Don't do anything.
@@ -231,7 +231,7 @@ class Posts extends Component {
       return <p>Loading...</p>
     }
 
-    let posts = this.props.posts.map(post =>
+    const posts = this.props.posts.map(post =>
       <Post post={post} key={post.id} />
     )
 
@@ -262,7 +262,7 @@ Consider the code above rewritten with [redux-thunk](https://github.com/gaearon/
 export function loadPosts(userId) {
   // Interpreted by the thunk middleware:
   return function (dispatch, getState) {
-    let { posts } = getState()
+    const { posts } = getState()
     if (posts[userId]) {
       // There is cached data! Don't do anything.
       return
@@ -315,7 +315,7 @@ class Posts extends Component {
       return <p>Loading...</p>
     }
 
-    let posts = this.props.posts.map(post =>
+    const posts = this.props.posts.map(post =>
       <Post post={post} key={post.id} />
     )
 
@@ -461,7 +461,7 @@ Redux reduces the boilerplate of Flux stores considerably by describing the upda
 Consider this Flux store:
 
 ```js
-let _todos = []
+const _todos = []
 
 const TodoStore = Object.assign({}, EventEmitter.prototype, {
   getAll() {
@@ -472,7 +472,7 @@ const TodoStore = Object.assign({}, EventEmitter.prototype, {
 AppDispatcher.register(function (action) {
   switch (action.type) {
     case ActionTypes.ADD_TODO:
-      let text = action.text.trim()
+      const text = action.text.trim()
       _todos.push(text)
       TodoStore.emitChange()
   }
@@ -487,7 +487,7 @@ With Redux, the same update logic can be described as a reducing function:
 export function todos(state = [], action) {
   switch (action.type) {
     case ActionTypes.ADD_TODO:
-      let text = action.text.trim()
+      const text = action.text.trim()
       return [...state, text]
     default:
       return state
@@ -506,7 +506,7 @@ Let's write a function that lets us express reducers as an object mapping from a
 ```js
 export const todos = createReducer([], {
   [ActionTypes.ADD_TODO]: (state, action) => {
-    let text = action.text.trim()
+    const text = action.text.trim()
     return [...state, text]
   }
 })