Question

What does the keyup() at the end of it mean?

$("input").keyup(function () {
      var value = $(this).val();
      $("p").text(value);
    }).keyup();

Answer 1

Thanks @nnnnnn Sir.

The code bind a keyup event to all inputs already belong to DOM and immediately trigger it for those inputs.

Now,

$("input").keyup(function () {
  var value = $(this).val();
  $("p").text(value);
});

above code bind the keyup event to input and last .keyup() makes an initial trigger to keyup.

You can rewrite above code also as following:

$("input").keyup(function () {
      var value = $(this).val();
      $("p").text(value);
    })

$('input').keyup(); // or $('input').trigger('keyup');

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Does it create an Infinite loop?

NO, It triggers just one time at page load. See here