Chai expectations and Sinon.js (test spies) example checking if an object contains a certain key/value pair

ajax-spec.js

it('will POST data in JSON format', () => {
  let promise = ajax('POST', '/somewhere', {foo: "bar"})
    .then((result) => {
      expect(result).to.equal('OK');
    });
  expect(requests[0].requestBody).to.equal(JSON.stringify({foo: "bar"}));
  requests[0].requestHeaders.should.have.property("Content-Type").with.length("application/json;charset=utf-8".length);
  assert.propertyVal(requests[0].requestHeaders, "Content-Type", "application/json;charset=utf-8");
  expect(requests[0].requestHeaders).to.have.property("Content-Type", "application/json;charset=utf-8");
  requests[0].respond(200, {}, 'OK');
  return promise;
})