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app.locals.lang = 'en';
app.locals.appName = 'HackHall';

The app.locals object can also be invoked like a function:

app.locals([
  author: 'Azat Mardan',
  email: 'hi@azat.co',
  website: 'http://proexpressjs.com'
]);

From

Cover of Pro Express.js

Note

app.locals for sitewide variables