Express res.format() Method

Posted in Express Tutorial

Updated on Feb 18, 2024

By Mari Selvan

👁️ 26 - Views

⏳ 4 mins

💬 1 Comment

Photo Credit to CodeToFun

🙋 Introduction

Express.js, a popular web application framework for Node.js, provides a variety of features for handling HTTP responses. One such powerful feature is the res.format() method, which enables dynamic content negotiation.

In this guide, we'll explore the syntax, use cases, and best practices of the res.format() method in Express.js.

💡 Syntax

The res.format() method in Express.js has a flexible and expressive syntax:

syntax.js

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res.format(object)

object: An object where the keys are the possible content types and the values are the corresponding handler functions or data.

❓ How res.format() Works

res.format() allows your Express.js application to respond with different content types based on the client's preferences. It inspects the Accept header in the request and selects the appropriate response format. This is particularly useful for supporting multiple data formats, such as JSON, XML, or HTML, based on the client's preferences.

example.js

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app.get('/api/data', (req, res) => {
  const responseData = {
    message: 'Hello, Express!'
  };

  res.format({
    'text/html': () => {
      res.send('<h1>Hello, Express!</h1>');
    },
    'application/json': () => {
      res.json(responseData);
    },
    'text/xml': () => {
      const xmlResponse = `<message>Hello, Express!</message>`;
      res.type('application/xml').send(xmlResponse);
    }
  });
});

In this example, the server responds with HTML, JSON, or XML based on the client's Accept header.

📚 Use Cases

API Responses with Multiple Formats:

Use res.format() to provide different response formats for API endpoints, allowing clients to request data in various formats.

example.js

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app.get('/api/data', (req, res) => {
  const responseData = {
    message: 'Hello, Express!'
  };

  res.format({
    'text/html': () => {
      res.render('index', { message: 'Hello, Express!' });
    },
    'application/json': () => {
      res.json(responseData);
    }
  });
});

Content Negotiation for Webpages:

Apply content negotiation to web pages, responding with HTML or JSON based on the client's preferences.

example.js

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app.get('/webpage', (req, res) => {
  res.format({
    'text/html': () => {
      res.render('webpage');
    },
    'application/json': () => {
      const jsonResponse = { content: 'This is a webpage in JSON format.' };
      res.json(jsonResponse);
    }
  });
});

🏆 Best Practices

Provide Fallbacks:

Include a default or wildcard handler in your res.format() calls to handle cases where the client doesn't explicitly specify a supported format.

example.js

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res.format({
  'text/html': () => {
    res.render('index');
  },
  'application/json': () => {
    res.json(responseData);
  },
  default: () => {
    res.status(406).send('Not Acceptable');
  }
});

Support Common Content Types:

Be sure to support common content types that clients may request, such as HTML, JSON, XML, and others relevant to your application.

example.js

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res.format({
  'text/html': () => {
    res.render('index');
  },
  'application/json': () => {
    res.json(responseData);
  },
  'text/xml': () => {
    const xmlResponse = `<message>Hello, Express!</message>`;
    res.type('application/xml').send(xmlResponse);
  }
});

🎉 Conclusion

The res.format() method in Express.js provides a powerful mechanism for dynamic content negotiation, allowing your application to respond with different formats based on client preferences. Leveraging this feature enhances the flexibility and usability of your API endpoints or web pages.

Now, armed with knowledge about the res.format() method, go ahead and implement dynamic content negotiation in your Express.js projects!