How to test a simply if-statement in a function with JASMINE

Could anybody help me to test this simple function please? I have to write a test in JASMINE for the function check_if_mobile_width(). After long serching I tried with Jasmine Spy but don't understand how it exactly works... I always get this error: "Expected spy addClass to have been called. (1)"

Function:

function check_if_mobile_width() {
if ($(window).width() < 979) {
    $.root_.addClass('mobile-view-activated')
} else if ($.root_.hasClass('mobile-view-activated')) {
    $.root_.removeClass('mobile-view-activated');
}}

TEST:

describe("desktop or mobile -- check_if_mobile_width()", function(){
it("should return the kind of the device", function(){

    check_if_mobile_width();
    spyOn($.root_, "addClass"); 
    spyOn($.root_, "removeClass");

    if($(window).width() < 979)
        expect($.root_.addClass).toHaveBeenCalled();

    else 
        expect($.root_.removeClass).toHaveBeenCalled();
    }); 
});