Is it necessary to set a Content-Type in Node.js?

Question

Just started playing with Node.js and after seeing a few examples I see that usually the Content-Type is set before returning some content.

Usually something like this for HTML:

res.writeHead(200, {'Content-Type': 'text/html'});
res.write(html);
res.end();

For image:

res.writeHead(200, {'Content-Type': 'image/png'});
res.write(img, 'binary');
res.end();

I read the docs for .write() and it says if no header is specified "it will switch to implicit header mode and flush the implicit headers"

With some testing I found I can just write one line like so:

res.end(html); // or
res.end(img);

These both work fine. I also tested with my local Apache server and when I viewed the headers being set when loading an image there was no Content-Type header set there.

Do I need to bother setting them? What situations or bugs might arise if I don't?

Answer 1

The Content-Type header is technically optional, but then you are leaving it up to the browser to essentially guess what type of content you are returning. Generally you should always specify a Content-Type if you know the type (which you probably do).

Answer 2

If you're using Express within your node app, then response.send(v) will implicitly pick a default content-type depending on the runtime type of v. More specifically, the behavior of express.Response.send(v) is as follows:

• If v is a string (and no content-type has already been set) then send Content-Type: text/html
• If v is a Buffer (and no content-type has already been set) then send Content-Type: application/content-stream
• If v is any other bool/number/object (and no content-type has already been set) then send Content-Type: application/json

Here's the relevant source code from Express: https://github.com/expressjs/express/blob/e1b45ebd050b6f06aa38cda5aaf0c21708b0c71e/lib/response.js#L141