unit testing - Spying on JQuery Selectors in Jasmine

Question

I am unit testing some JavaScript with Jasmine and wish to spy on (mock) an element of the DOM that is accessed by a jQuery selector.

My spec is:

it("should be able to mock DOM call", function() {

    spyOn($("#Something"), 'val').andReturn("bar");

    result = $("#Something").val();

    expect(result).toEqual("bar");

});

In my specrunner.html I have:

<input type="hidden" id="Something" value="foo" />

Unfortunately the spec fails with:

should be able to mock DOM call Expected 'foo' to equal 'bar'.

Answer 1

This line is wrong:

spyOn($("#Something"), 'val').andReturn("bar");

Jasmine's spyOn function expects two parameters. The first is an existing object. The second is a function name as a string. You are correctly passing in the function name as a string ("val") but you are not passing in an existing object as the first parameter.

$("#Something")

...is not an existing object. It is the result (the return value) of a jQuery selector. More specifically, it will return a jQuery object representing the matched nodes - kind of like an array of results.

$

...is an existing object.

$.fn

...is an existing object.

$("#Something")

...is not an existing object - it is the result of a jQuery selector.

This will work:

it("should be able to mock DOM call", function () {
    //spyOn($.fn, "val").andReturn("bar"); //pre-jasmine 2.0 syntax
    spyOn($.fn, "val").and.returnValue("bar"); //Jasmine 2.0 Syntax
    var result = $("#Something").val();
    expect(result).toEqual("bar");
});