foreach char in string

I'm trying to achieve the below Java selection in Javascript, and I'm a little stumped at the moment...

Java:

String input = inputField.getText();

foreach (char c: input.toCharArray){
    input = input.replace('a','b');
    input = input.replace('b','c');
    //and so on...
}
return input;
}

This is what I've tried:

var input = document.getElementById('inputField').value; //input field is textarea

for(var i = 0; i < input.length; i++){
    for(input.charAt(i)){
        input = input.replace('a','b');
        input = input.replace('b','c');
    }
}
return input;
}

I've also tried some different variations, such as disregarding the loop entirely and just using replace; however, then the replaces will just fire off sequentially and will not cover the length of long multi spaced strings.

Any help on this would be greatly appreciated. Thanks!