Jasmine: Test that an object is an instance of a given class

To test that an object was constructed by a given constructor function, use jasmine.any(Klass):

describe('plus()', function() {
  it ('returns a number', function() {
    let result = plus(1, 2)
    expect(result).toEqual(jasmine.any(Number))
  })
})

Also see Expecting objects as method invocation arguments.