unit testing - how to write a test for Javascript -

i attempting write test code having problems.

const attemptfinish = (taskid, then) => (disp) => { return api     .finishtask(taskid)     .then(then); }; export default attemptfinish; 

the finishtask function so

finishtask(taskid) {     return ajaclass         .post(host + '/comptask/mark', {             id: taskid         }); } 

i using sinon try , create response used results. has promise makes more complex

describe ("attemptfinish assess actions", () => { var participantid = 145; var prom = new promise((resolve, reject) => { resolve(); });  it("should return", () => {     var astub = sinon.stub(api, "finishtask")     astub.withargs(participantid, " ");     var callback = sinon.spy();      attemptfinish(participantid, prom)     astub.restore(); }); });