res.set() throws unexpected error when content-type is an array

[oztune]

When I run this code:

res.set('content-type', ['application/json'])`

The following error is thrown:

TypeError: value.split is not a function

It seems to stem from the following line which assumes that the value of content-type will always be a string:
https://github.com/expressjs/express/blob/master/lib/response.js#L721

Is this a bug or a feature?

EDIT: Please let me know what the expected behavior is and I will be happy to submit a PR. I'm assuming we should either handle this error, or accept an array for 'content-type'.

[dougwilson]

Hi @oztune I would say that accepting an array doesn't even make sense, and since this code has been there for all of 4.x, providing an array would have always been an error. I think that this is a good change, in that we should add a guard that throws a better error message when a non-string is provided for content-type.

My reasoning is that it's easier to loosen a restriction than to add it back later, so if there ever is some demand for an array, we can easily add that, but I can't think of any reason an array would make sense in this header, unless you know.

Please feel free to make a PR :) !

[oztune]

@dougwilson I agree, I'll make a PR that throws a more sensible error. My use-case was that I'm proxying a node-fetch response over to an express response, and a very special API error was causing this odd failure.