still learning JS and confused over this

the above code gives out “undefined” as output when it should be “v”
am i missing something or using for-in over a string is the issue

let s="iydafgvb";
var i = 0
for(i in s){
    if(s.charAt(i)=="a"){
        break;
    }
} 
console.log(s.charAt(i));

Expecting “v”
but getting “undefined”