I need a prompt to show just once, save a username, and use it in the rest of the site

First post. I am really new at Javascript, so I may be asking a lot around here. Well, the problem is this: The user enters the site, a prompt asks for their name and then, the DOM is edited to show a 'h3' saying "Welcome userX". But, the first problem was that, since the .js file was shared by every file of the site, it showed the prompt every time. To avoid this, I added the code in a function that is called when the body is loaded.

var salute = function(){
    var user = prompt("Por favor ingrese su nombre", "Usuario");
        if( user==null){
        user = "Usuari@ sin nombre"
    };

    var saludo = document.getElementById("saludo");

    saludo.innerHTML= "Bienvenido " + user; 

}

But, since I have this: <body onload="salute();"> Whenever the user goes back to the "Home" they are asked for its name again. How can I do to avoid this? And aslo, is there a way to use the "user" variable in another part of the site? For example, in an image gallery, make it to say "We hope you enjoy this pictures, user"

Solution

If you want to persist (localstorage) the username for next visits, and across other pages, then you may use this, and enhance it with additional validations:

var salute = function() {
  var user = localStorage.getItem("user");

  if (!user) {
    user = prompt("Por favor ingrese su nombre", "Usuario");
    if (user) {
      user = localStorage.setItem("user", user);
    } else {
      user = "Usuari@ sin nombre"; //Your default one
    }
  }

  var saludo = document.getElementById("saludo");
  saludo.innerHTML = "Bienvenido " + user;

}

Hope it works!