setsGets assertion helper for Chai

helper.js

module.exports = function (chai, utils) {
  var Assertion = chai.Assertion;
  
  Assertion.addMethod('setsGets', function (val) {
    
    var method = this._obj;

    // setter/getters must be functions
    new Assertion(method).to.be.a('function');
    
    // should be empty at first
    new Assertion(method()).to.not.exist;

    // see if the function will accept
    function setVal() { 
      method(val); 
    }

    // use a different set of assertions if we're negated
    if (utils.flag(this, 'negate')){
      new Assertion(setVal).to.throw(Error);
      new Assertion(method()).to.not.equal(val);
    }
    else {
      new Assertion(setVal).to.not.throw(Error);
      new Assertion(method()).to.equal(val);
    }
      
  });
};