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Setting the right value for the “Content-Type” header is a critical task for any web server

In Part III of this series, we built out our Node.js server to serve up any file that is requested, or gracefully return a 404 / “Not Found” header, as well as a custom HTML page informing the user that the requested file was not found. But the only problem is that we are not setting headers for each request. So, it’s important that HTML, CSS, JavaScript and image file, have the appropriate “Content-Type” header set.

In order to demonstrate all of this, we will include a CSS file and a JavaScript file in our web page. I won’t bother including the CSS file in an example; it’s just some silly CSS… nothing too interesting. But I will include the source of our JavaScript file in Example # 1, just so we can see that the second blue box in the web page is created via JavaScript.


Example # 1

Example # 1 simply shows the contents of the JavaScript file: “script.js” that we will request in our pages.

Example # 2

Example # 2 is the source code for index.html.

Example # 3

Example # 3 is the source code for about.html, a second web page that we can request from our Node.js web server.

Before we go any further, let’s take a  moment to discuss the folder structure. Just as in Part III of this series, the folder that our server.js file sits in has a sub-folder named “public”. This is where our assets will go. It is this “public” folder that the world will have access to and all requested files will be in there. See Figure # 1.

Folder structure
Figiure 1: The folder structure for this article’s example.

Example # 4

In Example # 4, we have a JavaScript object that contains a list of mime-types that we will support. Each of the object’s properties represents a file extension that we plan to support, and the corresponding value is the mime-type string that will be used for the “Content-Type” header.

Example # 5

In Example # 5, we have added a new line to the variable declarations for the requestHandler() function. The variable “ext” contains a string copy of the requested file’s extension (e.g. “.html” for a web page, “.js” for a JavaScript file, and so on). So we will use that string to check all properties of the “extensions” object from Example # 4. Don’t worry if you feel like you are getting lost; we’ll piece everything together nicely at the end. For now, just know that we have so far provided a hard-coded list of file extensions that we will allow, and the mime-type string values for each one, and we have the variable: “ext” that tells us what the file type is.

Example # 6

In Example # 6, we see if the extensions object has a property that matches the value of the “ext” variable. If not, we write a 404 header, and return a simple HTML page, informing the user that the requested file type is not supported.

Example # 7

In Example # 7, we add a new argument to the getFile() function call. We pass-in the value of the property in the extensions object that matches the file extension of the user’s request. In a nutshell, we are telling the getFile() function what type of mime-type to set in the response header.

Example # 8

In Example # 8, we have expanded the res.writeHead() function call. Where previously we only set the 200 / “OK” response code, we now set the “Content-Type” and “Content-Length” headers. The “Content-Type” property is mapped to the value of the mimeType variable, which was passed-in as an argument to the function. So the fruits of our labor in this article all become apparent in this example. The value of the mimeType variable will be set accordingly, for the file type.

Example # 9

Example # 9 is a complete code-listing for this article. In Figure # 2, we see the results of http://localhost:3000/index.html. So, as you can see, we are serving not only HTML, but also an image file, a CSS file and a JavaScript file. The JavaScript file dynamically creates the blue box you see on page load (simply to demonstrate that our JavaScript file is served correctly from our Node.js web server, and works).

index.html page
Figure # 2: Our index.html page.

In Figure # 3, we see that our “about.html” page works, and also pulls in the CSS and JavaScript files with no problems.

Figure # 3: about.html

In Figure # 4, we inspect the JavaScript file call in the “net” tab of FireBug and can see that the “Content-Type” header is set accordingly.

The FireBug
Figure # 4: Inspecting the call to script.js in FireBug’s “net” panel


In this article we learned how to set the appropriate “Content-Type” header for each request, based on the file extension of the requested file. We demonstrated how to use the extname() method of the path module, to return the extension of the requested file. We also applied some logic, to handle scenarios in which the requested file-type is not supported.

Helpful Links for the Node.js path module

Helpful Links for HTTP header fields

Helpful Links for mime-types