How to add a "?" after every 2 characters?

can someone help me with this,

i have a string....

var key='';
key+='aaabacadag';

Code (markup):

i want to make it like this

?aa?ab?ac?ad?ag

Code (markup):

TIA.

 

Here is what you want:

var key='';
var str = 'aaabacadag';

for(var i=0; i<str.length; i++){
    if(i%2==0){
        key += "?";
    }

    key += str.substr(i,1);
}

Code (JavaScript):

 

Nice solution s_ruben