Express res.locals Property

Posted in Express Tutorial

Updated on Oct 28, 2024

By Mari Selvan

👁️ 167 - Views

⏳ 4 mins

💬 1 Comment

Photo Credit to CodeToFun

🙋 Introduction

Express.js, a widely-used Node.js web application framework, provides developers with powerful features for building flexible and maintainable web applications. Among these features is the res.locals property, allowing you to define response-scoped variables that are accessible within a specific request-response cycle.

In this guide, we'll explore the syntax, usage, and best practices of res.locals to enhance your Express.js applications.

💡 Syntax

The syntax for using res.locals is straightforward:

syntax.js

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res.locals.name = value;

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

❓ How res.locals Works

The res.locals property in Express.js allows you to define variables that are specific to the current request-response cycle. These variables are available to middleware functions and route handlers within the same cycle, providing a convenient way to pass data between different parts of your application during the processing of a single request.

example.js

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// Middleware setting a response-local variable
app.use((req, res, next) => {
  res.locals.currentUser = req.user;
  next();
});

// Route handler accessing the response-local variable
app.get('/profile', (req, res) => {
  res.send(`Welcome, ${res.locals.currentUser.username}!`);
});

In this example, the currentUser variable is set in middleware and accessed in a subsequent route handler within the same request-response cycle.

📚 Use Cases

User Information:

Store user information in res.locals during authentication middleware for easy access in subsequent route handlers.

user-information.js

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// Middleware setting user information in res.locals
app.use((req, res, next) => {
  if (req.isAuthenticated()) {
    res.locals.currentUser = req.user;
  }
  next();
});

// Route handler using the user information
app.get('/profile', (req, res) => {
  res.send(`Welcome, ${res.locals.currentUser.username}!`);
});

Template Data:

Set template data in res.locals to dynamically provide information to your views.

template-data.js

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// Middleware setting template data
app.use((req, res, next) => {
  res.locals.siteTitle = 'My Express Site';
  next();
});

// Route handler using the template data
app.get('/', (req, res) => {
  res.render('index', { title: res.locals.siteTitle });
});

🏆 Best Practices

Limited Scope:

Use res.locals for data that is specific to the current request-response cycle. Avoid storing data that needs to persist across multiple requests.
limited-scope.js
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```
// Good practice: Using res.locals for response-specific data
app.use((req, res, next) => {
  res.locals.message = 'Hello from res.locals!';
  next();
});
```
Avoid Global Variables:

Resist the temptation to use res.locals as a replacement for global variables. Keep the scope limited to the current response.
avoid-global-variables.js
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```
// Bad practice: Using res.locals as a global variable
res.locals.globalData = 'This is accessible in all routes';
```

🎉 Conclusion

The res.locals property in Express.js is a powerful tool for managing response-scoped variables, allowing you to pass data seamlessly between middleware functions and route handlers within the same request-response cycle. By understanding its usage and following best practices, you can enhance the flexibility and maintainability of your Express.js applications.

Now equipped with knowledge about res.locals, go ahead and optimize your Express.js projects with efficient handling of response-specific variables!