javascript - Express4. What's the difference between app.locals, res.locals and req.app.locals?

Question

I am so confused while using express 4. I use express-generator to generate my project. And there are app.js in root and index.js in router file. However, the tutorial on internet about express are using router directly in app.js. So when I want to set some variables in index.js(in router file), I use app.locals, but it doesn't work. But when I change to the other two, my ejs template works... I am very confused. Anybody can tell me the difference between them and how to use correctly, please?

<!-- language: index.js in router file -->

    var app = require('express');
    var router = express.Router();

    ....

router.get('/', function(req, res, next) {
    var _user = req.session.user;
    if (_user) {
      //does't work!!
      //app.locals.user=_user;
      //I am not sure about which usage is correct below
      //1.
      req.app.locals.user = _user;
      //2.
      // res.locals.user=_user;
    }
}

<!-- language: lang-ejs -->

    <% if (user) { %>
      <li><a class="navbar-link">Welcome <%= user.name %></a>
      </li>
      <span>&nbsp;|&nbsp;</span>
      <li><a href="/logout" class="navbar-link" id="logoutBtn">Logout</a>
      </li>
      <% } else { %>
        <li><a href="#" class="navbar-link" data-toggle="modal" data-target="#signinModal">登录</a>
        </li>
        <span>&nbsp;|&nbsp;</span>
        <li><a href="#" class="navbar-link" data-toggle="modal" data-target="#signupModal">注册</a>
        </li>
      <% } %>

Answer 1

• The app.locals object is a JavaScript object, and its properties are local variables within the application.

  app.locals.title
  // => 'My App' 
  app.locals.email
  // => '[email protected]'
  

  Once set, the value of app.locals properties persist throughout the life of the application

• In contrast with res.locals properties that are valid only for the lifetime of the request. When you handle the route where you have a res object, you won't have an app object there and vice-versa for app.locals.

• You can access local variables in templates rendered within the application. This is useful for providing helper functions to templates, as well as app-level data. Locals are available in middleware via req.app.locals (see req.app)

  app.locals.title = 'My App';
  app.locals.strftime = require('strftime');
  app.locals.email = '[email protected]';
  

One picture from Node.js In Action book as below, describe the difference of app.local and res.local