JSFiddle - React, Tailwind, and code Playground

HTML

<input id="email" type="text" />
<input id="email_mask" type="text" />

JavaScript

document.getElementById('email').onchange=function() {
    email = this.value;
    
    var posArr = email.indexOf('@');
    
    var hidden = '';
    
    for(i=1;i<posArr;i++)
        hidden += '*';
    
    var mask = email.slice(0,1) + hidden + email.substr(email.indexOf('@')-1);
    
    document.getElementById('email_mask').value = mask;
}