請問我用js判斷用戶名和密碼為空，如何在判斷用戶名為空時使其返回登錄界面？

函數sub()

var userName = document . getelementbyid('用戶名')。價值；

if(userName = = ' '){ alert(' userName不能為空')；返回false}else返回true

& ltscript/& gt；

& ltform method = " post " id = " form 1 " name = " form 1 " action = ' log in . action ' on submit = " return sub()；"& gt& ltinput type = " text " name = " userName " id = " userName "/& gt；& lt/form & gt；