Cannot set property 'cache-control' of undefined in io.js/v0.12

[halfdan]

Not exactly sure what is happening here but express.js is failing on io.js and v0.12 when used within Ghost: https://travis-ci.org/halfdan/Ghost/builds/50824137

TypeError: Cannot set property 'cache-control' of undefined
      at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:341:22)
      at ServerResponse.header (/home/travis/build/halfdan/Ghost/node_modules/express/lib/response.js:695:10)
      at ServerResponse.header (/home/travis/build/halfdan/Ghost/node_modules/express/lib/response.js:698:12)
      at Object.errors.error404 (/home/travis/build/halfdan/Ghost/core/server/errors/index.js:290:13)
      at Context.<anonymous> (/home/travis/build/halfdan/Ghost/core/test/unit/error_handling_spec.js:342:20)

Here's a piece of code that shows the issue:

var express = require('express');
var res = express.response;
res.set({'cache-header': 'foo'});

Above code works in v0.10 but not in v0.12. Express version is latest 4.11.2.

ref #2539, #2538

[halfdan]

__proto__ is deprecated and probably not supported in either version anymore: https://github.com/strongloop/express/blob/master/lib/response.js#L29-L31

[dougwilson]

Hi! This is not an express bug, but simple a change between 0.10 and 0.12. express.response is just a subclass of the code Node.js require('http).ServerResponse.

As such, you must construct a new instance of the class to actually call methods on it. You should have been newing up a copy all this time, but as of 0.12 they added more stuff to the constructor so now you really need to new it up before you can interact with the headers.

As for that express.response export, even though you can certainly new it up and fix your issue, we don't even support you doing that; it's only there so you can attach your own methods to the Express response prototype, not to construct objects with.

This what you're doing, reduced down to Node.js core only. You can see it "works" in 0.10 (but of course, pollutes the global http response prototype), but not 0.12:

var http = require('http');
var res = http.ServerResponse;
res.setHeader('cache-header', 'foo');

[dougwilson]

TL;DR, your test mocking was just fragile enough to notice a change in Node.js core. I recommend not using mocking.

[dougwilson]

proto is deprecated and probably not supported in either version anymore

Um, __proto__ is part of the ES6 draft specification now... https://people.mozilla.org/~jorendorff/es6-draft.html#sec-__proto__-property-names-in-object-initializers But it's not relevant to this issue, which is that you need to var res = new express.response().

[lukesneeringer]

If you try to do var res = new express.response(), you get:

  - TypeError: express.response is not a function

[dougwilson]

This is a really old thread from many months ago. The proper solution is the following:

$ node -pe 'function Mock(){}; Mock.__proto__ = require("express").request; new Mock()'
{}