How jasmine spy example works - testing

Question

All;
I am just starting learning Jasmine( version 2.0.3 ), when I got to Spies section, the first example confused me:
describe("A spy", function() {
var foo, bar = null;
beforeEach(function() {
foo = {
setBar: function(value) {
bar = value;
}
};
spyOn(foo, 'setBar');
foo.setBar(123);
foo.setBar(456, 'another param');
});
it("tracks that the spy was called", function() {
expect(foo.setBar).toHaveBeenCalled();
});
it("tracks all the arguments of its calls", function() {
expect(foo.setBar).toHaveBeenCalledWith(123);
expect(foo.setBar).toHaveBeenCalledWith(456, 'another param');
});
it("stops all execution on a function", function() {
expect(bar).toBeNull();
});
});
I wonder if anyone could explain why the setBar function does not affect the bar defined inside describe block? How Jasmine spies deal with this?
Thanks

Answer 1

Because you are not actually executing the methods.
If you want this test to fail:
it("stops all execution on a function", function() {
expect(bar).toBeNull();
});
After these calls:
foo.setBar(123);
foo.setBar(456, 'another param');
Then you should call and.callThrough for your spy.
spyOn(foo, 'setBar').and.callThrough();
From the documentation
Spies: and.callThrough
By chaining the spy with and.callThrough, the spy will still track all
calls to it but in addition it will delegate to the actual
implementation.
With regard to your question, 'how jasmine deals with this?'
From here you can read a basic explanation:
Mocks work by implementing the proxy pattern. When you create a mock
object, it creates a proxy object that takes the place of the real
object. We can then define what methods are called and their returned
values from within our test method. Mocks can then be utilized to
retrieve run-time statistics on the spied function such as:
How many times the spied function was called.
What was the value that the function returned to the caller.
How many parameters the function was called with.
If you want all of the implementation details, you can check the Jasmine source code which is Open Source :)
In this source file CallTracker you can see how the gather data about the method calls.
A little more about the proxy pattern.