Make `.throw()` check error instance

[mstade]

This adds support for the following assertion, which didn't work before.

var expect = require('must')
   , err   = new Error('Sad face! :o(')

expect(function() { throw instance }).to.throw(err)

Love your coding style btw!

[moll]

Hey! Thanks for the help and sorry for a late reply.

So, on the surface a great addition. I'm just a little worried whether this will make Must.prototype.throw complicated. Currently it's a matcher-style function, with every input doing something special instead of its immediate value. Checking instances of Errors directly would diverge to a special case.

I myself almost never use must.throw and instead do a bit of boilerplate:

var ex
try { someFn() } catch (ex) { err = ex }
ex.must.be.an.instanceof(HttpError)
ex.code.must.equal(404)
ex.message.must.include("Not Found")

That to me is more expressive and generic, especially if you want to test other properties besides message. In that vein, I've been instead thinking of perhaps a helper to simplify that catch block. Perhaps something like:

var err = Must.catch(someFn)
ex.must.be.an.instanceof(HttpError)
ex.code.must.equal(404)

That Must.catch could also be named Must.throw or Must.thrown etc. Your use case would then be something like Must.catch(someFn).must.equal(yourErr).

What do you think?

[mstade]

Oh I like that, good idea! It'd also enable this nifty pattern (which Crockford-ites will hate):

with (Must.catch(someFn)) {
  expect(message).to.equal('oh noes')
  expect(code).to.equal(404)
}

Now, I guess Must and expect are in fact equivalent here, so Must.catch could be expect.error or expect.thrown or something. But that's all bike shedding anyway. It's a good idea for sure. Unfortunately, I don't have time to implement it right now; if I can find a spare moment I'll see what I can do.

[moll]

Thanks for the quick response.

Don't worry about it. I can do that once I get in the flow. ;-) I've been meaning to refactor those tests for a while now. They're a little too verbose. Not to mention other issues here need some additions.

[moll]

I added #19 to continue the expect.error idea. Should we close this for now and later return to it when applicable?

[mstade]

Sure thing!