Express res.headersSent Property

Posted in Express Tutorial

Updated on Feb 18, 2024

By Mari Selvan

👁️ 26 - Views

⏳ 4 mins

💬 1 Comment

Photo Credit to CodeToFun

🙋 Introduction

In Express.js, managing HTTP headers is crucial for controlling the behavior of your web application. The res.headersSent property is a valuable tool that allows you to check whether headers have been sent in the HTTP response.

In this guide, we'll explore the syntax, use cases, and best practices for utilizing the res.headersSent property in your Express.js applications.

💡 Syntax

The res.headersSent property can be accessed directly from the response object:

syntax.js

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if (res.headersSent) {
  // Headers have been sent
} else {
  // Headers have not been sent
}

❓ How res.headersSent Works

The res.headersSent property is a boolean value that indicates whether headers have been sent in the HTTP response. This is particularly useful when you need to conditionally set headers based on certain conditions or ensure that headers are not modified after being sent.

example.js

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app.get('/example', (req, res) => {
  if (!res.headersSent) {
    // Set headers if not already sent
    res.setHeader('Content-Type', 'text/plain');
  }
  res.send('Hello, Express!');
});

In this example, the res.headersSent property is used to conditionally set the 'Content-Type' header before sending a response.

📚 Use Cases

Conditional Header Setting:

Use res.headersSent to conditionally set headers based on certain criteria before sending a response.

conditional-header-setting

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app.get('/conditional', (req, res) => {
  if (!res.headersSent) {
    // Set headers based on a condition
    if (req.query.format === 'json') {
      res.setHeader('Content-Type', 'application/json');
    } else {
      res.setHeader('Content-Type', 'text/plain');
    }
  }
  res.send('Conditional Header Setting');
});

Preventing Header Modification:

Leverage res.headersSent to prevent unintentional modification of headers after they have been sent.

preventing-header-modification.js

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app.get('/immutable', (req, res) => {
  // Check if headers are already sent before attempting to modify
  if (!res.headersSent) {
    // Set headers
    res.setHeader('Cache-Control', 'no-store');
  }
  // Attempting to modify headers after they are sent will throw an error
  res.send('Immutable Headers');
});

🏆 Best Practices

Early Check for Headers:

Perform checks for res.headersSent early in your route handler to ensure headers are set or modified before being sent.

early-check-for-headers

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app.get('/bestpractice', (req, res) => {
  // Early check for headers
  if (!res.headersSent) {
    // Set or modify headers as needed
    res.setHeader('Content-Type', 'text/html');
  }
  res.send('Best Practice Example');
});

Use Conditional Statements:

Employ conditional statements with res.headersSent to dynamically adjust header settings based on application logic.

use-conditional-statements.js

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app.get('/conditionalbest', (req, res) => {
  if (!res.headersSent) {
    // Set headers conditionally
    res.setHeader('X-Powered-By', 'Express');
    if (req.query.debug === 'true') {
      res.setHeader('Debug-Mode', 'enabled');
    }
  }
  res.send('Conditional Best Practice');
});

🎉 Conclusion

The res.headersSent property in Express.js is a powerful tool for managing HTTP headers effectively. Whether you need to conditionally set headers or prevent unintended modifications, understanding how to leverage res.headersSent is crucial for fine-tuning your Express.js applications.

Now, equipped with knowledge about res.headersSent, go ahead and enhance the control over your HTTP headers in your Express.js projects!