Java script comparing two variables

There must be some thing there - the following two issues must be related to one reason, I spend two days to understand why

ISSUE 1
        var y2 = 2000;
        alert(y2);   (this line gives alert 2000 which means y2 value is 2000)
        alert(y2 == 2000);   (this line gives alert true which also means y2 value is 2000)

        if (y2 = 2000 ) {
            alert("y2 is equals 2000");    (never get to this alers !!!!!     any Idea?)
            return false;
        }

ISSUE 2
assuming textbox1.text="11-1-2000"
assuming textbox2.text="11-1-2001"

        var T1= document.getElementById("<%=Textbox1.ClientID%>");
        var year1 = T1.value.split("-")[2];
        var T2= document.getElementById("<%=Textbox2.ClientID%>");
        var year2 = T2.value.split("-")[2];
       
        alert(year1);   (this line gives alert 2000 which means T1 value is 2000)
        alert(year2);   (this line gives alert 2001 which means T2 value is 2001)
       
        if (year1 > Year2)
        {
            Alert("Year1 is GT Year2")   (never get to this alers !!!!!     any Idea?)
            return false;
        }