express-handlebars如何向模板传全局变量？

学习express.js中，使用了express-handlebars引擎，现在想向模板里传一些全局变量，看了
npm里 express-handlebars 的文档发现一些相关信息：

Metadata Handlebars has a data channel feature that propagates data
through all scopes, including helpers and partials. Values in the data
channel can be accessed via the {{@variable}} syntax. Express
Handlebars provides metadata about a template it renders on a
{{@exphbs}} object allowing access to things like the view name passed
to res.render() via {{@exphbs.view}}.

The following is the list of metadata that's accessible on the
{{@exphbs}} data object:

cache: Boolean whether or not the template is cached.
view: String name of the view passed to res.render().
layout: String name of the layout view.
data: Original data object passed when rendering the template.
helpers: Collection of helpers used when rendering the template.
partials: Collection of partials used when rendering the template.

想请教一下，这个用法是怎样的？或者有别的方式？十分感谢！

---

实话说，metadata这段我没怎么看懂。不过设置全局变量我倒是知道一点点：

var app = express();

//设置全局变量sayHi
app.locals.sayHi = 'Hello World!';

app.engine('handlebars', exphbs({defaultLayout: 'main'}));
app.set('view engine', 'handlebars');

app.get('/', function(req, res) {
    res.render('home');
});

<h1>Example App: Home</h1>
<p>
    {{sayHi}}
</p>

这里使用sayHi变量