I've been searching the internet all day now, but I just can't find the thing I am looking for (might be language barrier). So I really hope one of you might be able to help me with this.

I'm using the following example to show what I want to do with html & js.

I have 3 Buttons and when clicked they do the buttoninput function in js.

 a 
 b 
 c 
 this one is so that it won't be an endless input 

in javascript

let inputvalue = ""; 

function buttoninput(button){
    inputvalue = button.value; 
    //But what do I need to add here so it's not only a single value?
    console.log(inputvalue);
}

But this is where I am getting stuck. I do get the right input, for example button "a". But it's only ever one single letter.

But what if I want to do a combo that goes "aba"?

I would be really gratefull if anyone could help me here or lead me in the right direction.