Global Data Context

Passing static configuration data (like a site title or contact email) to every single res.render call violates the “Don’t Repeat Yourself” (DRY) principle. Express provides a mechanism to define global variables that are automatically available to every view in the application, eliminating the need to manually transport them through each route.

The app.locals Object

The app.locals object is a JavaScript object whose properties are local variables within the application. Once set, these persist throughout the life of the application.

Correct Use Cases:

• Global configuration (Site Name, API keys for client-side scripts).
• Helper functions (Formatters, utility methods).
• Static data (Copyright year, contact info).

Incorrect Use Cases:

• User Sessions: Do not store user-specific data here. app.locals is shared across all requests. Storing a user here would make that user visible to everyone. Use res.locals for request-specific data.

Implementation

File: app.js

const express = require("express");
const app = express();

// Configuration: Set global application variables
// These are available in ALL views automatically
app.locals.appTitle = "Node2Know";
app.locals.adminEmail = "ps@professorsolo.com";
app.locals.license = "Node2Know Learner License 1.0 (Node2Know-LEARN-1.0)";
app.locals.copyrightYear = new Date().getFullYear();

// Route Handler
app.get("/", (req, res) => {
  // No need to pass 'appTitle' here; it's already in the context
  res.render("index");
});

Usage in Views

In your EJS templates, these properties are accessed as top-level variables. You do not need to prefix them with app.locals.

File: views/partials/footer.ejs

<footer>
  <p>
    &copy; <%= copyrightYear %> Josh Solomon - Professor Solo - <%= appTitle %>
  </p>
  <p>License: <%= license %></p>
</footer>

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