JSFiddle - React, Tailwind, and code Playground

by dbugger

JavaScript

function mask(str, i){
    str1 = str.substring(0, str.length - i).replace(/./gi, "X");
    str2 = str.substring(str.length - i);
    
    return str1 + str2;
}

document.write( mask("123456789", 4) );