JSFiddle - React, Tailwind, and code Playground

JavaScript

function changeMail(str) {
    var split = str.split("@");
    var letter1 = split[0].substring(0, 1);
    var letter2 = split[0].substring(split[0].length - 1, split[0].length);
    var newFirst = letter1;
    for(i = 0; i < split[0].length - 2; i++) {
        newFirst += "*";
    }
    newFirst += letter2;
    
    var letter3 = split[1].substring(0, 1);
    var extension = letter3;
    for(i = 0; i < split[1].split(".")[0].length - 1; i++) {
        extension += "*";
    }
    extension += "." + split[1].split(".")[1];
    var result = newFirst + "@" + extension;
    
    return result;
}

console.log(changeMail("[email protected]"));