Chai: how can I check exception properties

Question

Consider a piece of code below:

var expect = require('chai').expect;

parse = function() {
  throw {error: 'PROTOCOL_ERROR'}
}

describe('parser', function() {

  it('throw error on unfinished command', function() {
    try {
      parse("*2\r\n$4\r\necho\r\n$11\r\nhel")
    } catch (e) {
      expect(e).to.equal({error: 'PROTOCOL_ERROR'})
    }
  })
});

Is there a way to avoid this try...catch boilerplate in the code? Chai has 'throw' method but it provides only an instanceof check

score 5 · Answer 1 · answered Apr 22 '16 at 10:50

I found this answer:

var expect = require('chai').expect;

parse = function() {
  throw {error: 'PROTOCOL_ERROR'}
}

describe('parser', function() {
  it('throw error on unfinished command', function() {
      expect(
        () => parse("*2\r\n$4\r\necho\r\n$11\r\nhel")
      ).to.throw().which.has.property('error', 'PROTOCOL_ERROR');
  })
});

aray12 · Answer 2 · 2016-04-22T16:05:14.570

0

First, you should be throwing an instance of Error or something that extends Error

parse = function() {
  throw new Error('PROTOCOL_ERROR');
}

Then chai has a method to assert that a function is throwing an error http://chaijs.com/api/bdd/#method_throw

So the final thing should look like this:

var expect = require('chai').expect;

parse = function() {
  throw new Error('PROTOCOL_ERROR');
}

describe('parser', function() {

  it('throw error on unfinished command', function() {

    expect(function() { parse("*2\r\n$4\r\necho\r\n$11\r\nhel") }).to.throw(Error, 'PROTOCOL_ERROR');

  })
});

Edit: wrapped function call in function. Otherwise the error is thrown before expect is called and has a chance to catch it.

edited Apr 22 '16 at 16:05

answered Apr 21 '16 at 20:42

aray12

1,833
1
16
22

You can only assert that the error was thrown using a specific constructor and what the message is. What property are you trying to check? – aray12 Apr 21 '16 at 20:56
No matter what the property. look for example: http://stackoverflow.com/a/24078053/1907902 it can be sMessage, for example. – kharandziuk Apr 21 '16 at 21:06
Then you can't avoid the boilerplate above using chai. Just a warning, if you write it with a `try{}catch{}` and it doesn't throw nothing will be asserted and it will pass – aray12 Apr 21 '16 at 21:17
yep, you understand the problem now:) – kharandziuk Apr 21 '16 at 21:18
Thanks for you answer – kharandziuk Apr 21 '16 at 21:19
Hey, I think your sample is incorrect. The test doesn't pass – kharandziuk Apr 22 '16 at 09:01
Can you include the expected and actual values it provides? – aray12 Apr 22 '16 at 15:46
@kharandziuk I edited my answer slightly. You pass the function without invoking it to expect. Otherwise it is called right away without giving it a chance to catch it – aray12 Apr 22 '16 at 16:06

Chai: how can I check exception properties

2 Answers2