Stubbing a constructor with parameters using sinon

Question

I have the following constructor which takes a few parameters.

constructor(url: string, mqttOptions: MqttOptions, messageReceivedCallBack: IMessageReceivedCallBack) {
  if (!_.isString(url) || _.isEmpty(mqttOptions || _.isEmpty(messageReceivedCallBack))) {
  throw new MyNodeError('invalid url value');
} else { 
       this.createConnection(url,mqttOptions);
}
}

How can I create a spy instance using sinon in order to verify that an exception is thrown when one of the parameters is empty? I saw this question Mocking JavaScript constructor with Sinon.JS but its a constructor with no parameters. Any help would be much appreciated.

score 1 · Answer 1 · answered May 20 '16 at 09:53

I don't see why you need to create a spy for that? You should be able to just test it like so:

it('should throw if one of the parameters is empty', function() {
    expect(constructor.bind(null, null, mqttOptions, messageReceivedCallback)).to.throw(MyNodeError);
}

In the example url parameter is set to null and I'm assuming the variables mqttOptions and messageReceivedCallback to be defined (not shown).

This, for example, mirrors what you're looking for (if I understood the question correctly) and works as expected for me:

it.only('', function() {
    function C(a) { if (!a) throw new TypeError('a was empty'); }
    expect(C.bind(null, 'b')).to.not.throw(TypeError);
    expect(C.bind(null, null)).to.throw(TypeError);
})

score 1 · Accepted Answer · answered May 20 '16 at 09:54

Try whether the following approach works for you!

import {assert} from 'chai';
import MyNodeError from './path/to/MyNodeError'
import ClassToTest from './path/to/Class';

describe('MyClass', function() {
    function test() {
       new ClassToTest('', {}, null);
    }
    it("Should throw exception when invalid parameters passed", function(){
        assert.throw(test, MyNodeError, "invalid url value");
    });
});

Stubbing a constructor with parameters using sinon

2 Answers2