How to send string variables as parameter for Ajax call?

The following Ajax call would send to PHP the parameters corresponding to the form that called the function:

var foo = "bar";
var bar = "foo";

function register(){
  $.ajax({
    method: "post",
    url: "meu_script.php",
    data: $("#form").serialize(),
  });
}

And it would be accessible in PHP through id s of HTML form components:

$campo = $_POST['id_campo'];
$senha = $_POST['id_senha'];

My question: what if instead of using a form, I wanted to pass the values of the variables foo and bar , what would be the data parameter of my Ajax call?

Just specify a javascript object:

var foo = "bar";
var bar = "foo";

function register(){
  $.ajax({
    method: "post",
    url: "meu_script.php",
    data: {foo: foo, bar: bar},
  }); 
}

To get a little less confused, I would give a different name to the variables. Something like this:

var _foo = "bar";
var _bar = "foo";

function register(){
  $.ajax({
    method: "post",
    url: "meu_script.php",
    data: {foo: _foo, bar: _bar},
  }); 
}

Use $.post documentation: link

jQuery

var foo = "bar";
var bar = "foo";
$.post('meu_script.php',{
  parametro_qualquer_nome1:foo,
  parametro_qualquer_nome2:bar
 },function(){
  // ação quando for concluída a solicitação 
})

PHP

$foo = $_POST['parametro_qualquer_nome1'];
$bar = $_POST['parametro_qualquer_nome2'];