Sinon spy fails with bad arguments for one function but succeeds for another identical one

I am really confused by the behaviour I'm seeing. My spy keeps misreporting the arguments. If I create identical functions with the same signature and arguments and spy on that separately then the copy works fine. I cannot work out what's going on!

So, here's the code:

NB: alert is the original function, test is the new one I created to check what's going on.

# index.js
class Alerter {

  getOnErrorFn(name) {
    return (error) => {
      ...
       alert = {
         opts: {
           tags: 'Unknown Error'
         }
       }
       ...

      if (alert) {
        this.test(name, error.message, Object.assign({}, alert.opts, {smc_error: error.toLog()}), alert.level);
        this.alert(name, error.message, Object.assign({}, alert.opts, {smc_error: error.toLog()}), alert.level);            }
    };
  }

  test(serviceName, message, opts={}, level=this.CRITICAL) {
    console.log(serviceName, message, opts, level);
  }

  alert(serviceName, message, opts={}, level=this.CRITICAL) {
    console.log(serviceName, message, opts, level);
    ...
  }

Here's my test code (All other tests are commented out and this is the only file in the test suite);

# alerter.spec.js
const sandbox = sinon.sandbox.create();

describe('Alerter', function(){

  let alerter;
  let name = 'test-service';
  let message = 'test message';

  beforeEach(() => {
    alerter = new Alerter(name);
  });

  afterEach(() => {
    sandbox.restore();
  });

  describe('getOnErrorFn()', () => {
    it.only('handles non-SMCError errors and assumes they should be alerted', () => {
      const spy = sandbox.spy(alerter, 'test');
      const spi = sandbox.spy(alerter, 'alert');
      const onError = alerter.getOnErrorFn(name);

      const error = new Error();
      const smcError = SMCError.from(error);
      onError(error);
      expect(spy).to.have.been.calledWith(name, smcError.message, {smc_error: smcError.toLog(), tags: 'Unknown Error'}, undefined);
      expect(spi).to.have.been.calledWith(name, smcError.message, {smc_error: smcError.toLog(), tags: 'Unknown Error'}, undefined);
    });

  });

});

And here's the result of the test run.... which is doing my absolute nut in!

$ npm test

> smc-alerting@2.1.3 test /Users/al/Studio/Projects/smc/app/smc-alerting
> mocha test/**/*.spec.js



  Alerter
    getOnErrorFn()
TEST test-service Caught Error { tags: 'Unknown Error',
  smc_error: 'Error Caught Error\n  caused by: Caught Error' } critical
ALERT test-service Caught Error { tags: 'Unknown Error',
  smc_error: 'Error Caught Error\n  caused by: Caught Error' } critical
      1) handles non-SMCError errors and assumes they should be alerted


  0 passing (34ms)
  1 failing

  1) Alerter getOnErrorFn() handles non-SMCError errors and assumes they should be alerted:
     expected alert to have been called with arguments test-service, Caught Error, {
  smc_error: "Error Caught Error
  caused by: Caught Error",
  tags: "Unknown Error"
}, undefined
    alert(test-service, Caught Error, { smc_error: "Error Caught Error
  caused by: Caught Error" }, undefined) at /Users/al/Studio/Projects/smc/app/smc-alerting/src/index.js:46:14
  AssertionError: expected alert to have been called with arguments test-service, Caught Error, {
    smc_error: "Error Caught Error
    caused by: Caught Error",
    tags: "Unknown Error"
  }, undefined
      alert(test-service, Caught Error, { smc_error: "Error Caught Error
    caused by: Caught Error" }, undefined) at src/index.js:46:14
      at Context.it.only (test/index.spec.js:173:32)



npm ERR! Test failed.  See above for more details.

So, note that both console.log statements print identical results. BUT, the spy for the alert function fails with a printout that indicates that the function was called with the tags property missing from the third argument. WTF!

Or is there something I don't know happening here?

All help much appreciated. Thanks in advance!

via Alastair Brayne