How to use the busboy.prototype function in busboy

pipe: function (s) {
                s.end('abc123');
                // ensure 'finish' event fires after files are processed
                process.nextTick(Busboy.prototype.on.withArgs('finish').args[0][1]);
            },
            truncated: true

afterEach(function () {
        Busboy.prototype.on.restore();
    });

beforeEach(function () {
        parser = bodyparser();
        req = {
            headers: {
                'content-type': 'multipart/form-data; boundary=----WebKitFormBoundaryHsAjApiShDrW2RCB',
            },
            is: sinon.stub().returns(true),
            pipe: sinon.stub()
        };
        res = {};
        next = sinon.stub();
        sinon.stub(Busboy.prototype, 'on');
    });

attr: true
              }
            })
            .concat(values(errors))
          if (validationErrors.length) {
            Busboy.prototype.emit.call(
              this,
              'error',
              createValidationError(formatRamlErrors(validationErrors, 'form'))
            )

            return
          }
        }

        return Busboy.prototype.emit.apply(this, arguments)
      }
    }