This repo reproduces a bug in Jest. The tests pass in Mocha but fail in Jest.

$ npm install
$ npm start &
$ npm run test:mocha
$ npm run test:jest

I have a test suite written in Mocha that makes requests to an endpoint and makes some assertions about the cookies in the HTTP response.

To make these requests, I'm using the node-fetch package, which internally just calls node's http.request.

As an example, the raw headers from http.request look like this:

{
    "x-powered-by": "Express",
    "set-cookie": [
        "test=foo; Path=/"
    ],
    "content-type": "text/html; charset=utf-8",
    "content-length": "19",
    "etag": "W/\"13-1sRJGKLXRCQyY8QPKPqBEg\"",
    "date": "Mon, 09 Jan 2017 16:03:17 GMT",
    "connection": "close"
}

node-fetch does several conditional checks to determine how to parse each response header. In the case of the set-cookie header, it should match the array condition which checks if the header is an instanceof Array.

In Mocha, this check succeeds and node-fetch parses the cookies header just fine.

In Jest, this check fails saying the set-cookie header is not an instanceof Array.

Array.isArray succeeds in both Mocha and Jest, so I've submitted a PR to update node-fetch to do so.