Uncaught TypeError: Cannot read property 'value' of undefined

First, you should make sure that document.getElementsByName("username")[0] actually returns an object and not "undefined". You can simply check like

if (typeof document.getElementsByName("username")[0] != 'undefined')

Similarly for the other element password.

Seems like one of your values, with a property key of 'value' is undefined. Test that i1, i2and __i are defined before executing the if statements:

var i1 = document.getElementById('i1');
var i2 = document.getElementById('i2');
var __i = {'user' : document.getElementsByName("username")[0], 'pass' : document.getElementsByName("password")[0] };
if(i1 && i2 && __i.user && __i.pass)
{
    if(  __i.user.value.length >= 1 ) { i1.value = ''; } else { i1.value = 'Acc'; }

    if(  __i.pass.value.length >= 1 ) { i2.value = ''; } else { i2.value = 'Pwd'; }
}

Either document.getElementById('i1'), document.getElementById('i2'), or document.getElementsByName("username")[0] is returning no element. Check, that all elements exist.

Try this, It always works, and you will get NO TypeError:

try{

    var i1 = document.getElementById('i1');
    var i2 = document.getElementById('i2');
    var __i = {'user' : document.getElementsByName("username")[0], 'pass' : document.getElementsByName("password")[0] };
    if(  __i.user.value.length >= 1 ) { i1.value = ''; } else { i1.value = 'Acc'; }
    if(  __i.pass.value.length >= 1 ) { i2.value = ''; } else { i2.value = 'Pwd'; }

}catch(e){
    if(e){
    // If fails, Do something else
    }
}