Express app.locals Property

Posted in Express Tutorial

Updated on Feb 18, 2024

By Mari Selvan

👁️ 46 - Views

⏳ 4 mins

💬 1 Comment

Photo Credit to CodeToFun

🙋 Introduction

Express.js, a popular Node.js web application framework, provides developers with powerful tools to build robust and scalable web applications. One such feature is the app.locals property, which allows you to define variables accessible throughout your application's lifecycle.

In this guide, we'll explore how to use app.locals effectively, covering its syntax, usage, and best practices.

💡 Syntax

The syntax for using app.locals is straightforward:

syntax.txt

Copied

app.locals.name = value;

name: The name of the variable you want to define.
value: The value assigned to the variable.

❓ How app.locals Works

The app.locals property in Express.js provides a way to define application-level variables that are accessible in all middleware and route handler functions. These variables are available throughout the entire lifecycle of the application, making them ideal for storing data that needs to be shared across different parts of your application.

example.js

Copied

// Define a global variable
app.locals.siteName = 'My Express App';

// Access the global variable in a route handler
app.get('/', (req, res) => {
  res.send(`Welcome to ${app.locals.siteName}`);
});

In this example, the siteName variable is defined using app.locals and is accessible within the route handler function.

📚 Use Cases

Configuration Settings:

Use app.locals to store configuration settings that are used throughout your application, such as port numbers or database URLs.
configuration-settings.js
Copied
```
// Define global configuration settings
app.locals.port = 3000;
app.locals.databaseUrl = 'mongodb://localhost/my_database';
```
Shared Data:

Store shared data, such as user information or application-wide constants, using app.locals.
shared-data.js
Copied
```
// Define global data
app.locals.users = [
  { id: 1, name: 'John' },
  { id: 2, name: 'Jane' },
  // more users...
];
```

🏆 Best Practices

Limit Global Variables:

Avoid cluttering app.locals with too many variables. Only store data that needs to be accessed globally across your application.
limit-global-variables.js
Copied
```
// Good practice: Limit global variables to essential data
app.locals.siteName = 'My Express App';
app.locals.port = 3000;
```
Immutable Variables:

Prefer immutable variables in app.locals to prevent unintended modifications.
immutable-variables.js
Copied
```
// Immutable variable
app.locals.siteName = 'My Express App';
```

🎉 Conclusion

The app.locals property in Express.js provides a convenient way to define and access global variables in your application. By following best practices and using app.locals effectively, you can streamline your development process and build more maintainable Express.js applications.

Now, armed with knowledge about app.locals, go ahead and leverage this powerful feature in your Express.js projects!