What is the correct JSON content type?

JSON (JavaScript Object Notation) and JSONP ("JSON with padding") formats seems to be very similar and therefore it might be very confusing which MIME type they should be using. Even though the formats are similar, there are some subtle differences between them.

So whenever in any doubts, I have a very simple approach (which works perfectly fine in most cases), namely, go and check corresponding RFC document.

JSON RFC 4627 (The application/json Media Type for JavaScript Object Notation (JSON)) is a specifications of JSON format. It says in section 6, that the MIME media type for JSON text is

application/json.

JSONP JSONP ("JSON with padding") is handled different way than JSON, in a browser. JSONP is treated as a regular JavaScript script and therefore it should use application/javascript, the current official MIME type for JavaScript. In many cases, however, text/javascript MIME type will work fine too.

Note that text/javascript has been marked as obsolete by RFC 4329 (Scripting Media Types) document and it is recommended to use application/javascript type instead. However, due to legacy reasons, text/javascript is still widely used and it has cross-browser support (which is not always a case with application/javascript MIME type, especially with older browsers).

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A part of your question is relevant to me as I just came across it.

A third-party provider is providing a REST service that is used by multiple clients of his. It's a straight-forward REST call with query parameters that returns a well-formed JSON. I tested it with PHP and Java where it worked as expected.

My client uses Oracle Service Bus as a gateway between his application server and the Internet. When I made the OSB service, it crashed with an "Invalid message format" error. Turned out that the content type being returned was "text/html". OSB treats responses as per this header; converting between text, XML and JSON. In this case, the response was JSON but the header didn't say so. Contacting the provider, I got the reply: "We're not going to change it as it doesn't effect anyone else".

The content type header specifies what the content should be, not what it actually is. That is to say, in your consuming program, it's up to you to check or ignore it and process the content in any manner. Another example, you can return GIF data but specify the content type as JSON, then go ahead and ignore the header and read the image data. This won't hurt your program, but may hurt others.

Moral of the story: Play nice.