Cannot get the value of datepicker of JQuery

Question

I'm trying to use datepicker of Jquery, but it seems I cannot catch the selected value of my datepicker.

My added HTML is:

test Date:<input type="text" id="testDate" placeholder="yyyy-mm-dd"/>
<input type="button" id="testbtn" value="my button"/>

in my javascript inside

$(document).ready(function() { 

I have the following code:

$("#testDate").datepicker({dateFormat: "yy-mm-dd"});
$('#testbtn').click(function() {
    console.log($('#testDate').val());
});

but nothing is written in my console! Am I missing something? Please let me know if you need more clarification.