I've got datas from an external source that uses initials to refer to specific type of data

I want to be able to get back the value. As I know the value for each shortcuts, I could do something like that :

function getValue(initial)
{
    if(initial == "A") return "Attelé";
    else if(initial == "AB") return "Abajour";
    else if(initial == "B") return "Base";
}

But this is not a good practice as I will have to add a new comparaison for every initial

So i'm thinking about using two array as references :

var values = ["Attelé","Abajour","Base"];
var initialList = ["A", "AB","B"];

function getValue(initial)
{
    var index = initialList.indexOf(initial);
    if(index > -1) return values[index];
    else return null;
}

console.log(getValue("A"));

But here again, i think there is a better practice. To add a new initial, I still have to modify two variables, and if my arrays's length are not matching I could have an issue with the result. I would like to be able to change only one variable.

Hence my question : How can I use one object/array/table to match my initials, and what is the best practice to look at ?