สอบถามเรื่องการใช้เงื่อนไข if else ใน javascript ไม่แสดงผลค่ะ

<script language="javascript" type="text/javascript">  
function useif(){  
    var t = parseFloat(document.frmMain.txttype_code.value); 
  if(t = "STD-P")  
        var t1=parseFloat(document.frmMain.pack_qty.value);
var t2=parseFloat(document.frmMain.txtpk_weight1.value);
document.frmMain.std_pur.value=(t1-t2)*t1;
 if(t = "STD+P")  
        var y1=parseFloat(document.frmMain.pack_qty.value);
var y2=parseFloat(document.frmMain.txtpk_weight1.value);
document.frmMain.std_pur.value=(y1-y2)*y1;
 if(t = "ENT-P")  
   var g1=parseFloat(document.frmMain.suppweight_total.value);
        var g2=parseFloat(document.frmMain.pack_qty.value);
var g3=parseFloat(document.frmMain.txtpk_weight1.value);
document.frmMain.std_pur.value=(y1-y3)*y2;

}  
</script>