example of stub with sinon

stub_with_sinon.js

// install sinon via npm
// npm install sinon --save-dev

// require sinon in your test/client_api.js
var sinon = require("sinon");

// add this test expectation
it("cover error", function (done) {

  var url = URL_ROOT + "/client";
  // stub client, changing return of Client.find
  stubClient = sinon.stub(Client, 'find', function(obj, callback) {
      callback(new Error('An Error Has Occurred'), []);
  });

  superagent.get(url)
      .set("Authorization", token)
      .end(function (error, res) {
         // make sure of call restore to avoid, unexpected result for others tests expectation
          stubClient.restore();
          assert.equal(res.status, status.INTERNAL_SERVER_ERROR);
          done();
      });
});