How can I make a browser display all datalist options when a default value is set?

Question

I have an HTML form with a datalist and where the value is set with PHP, like

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Answer 1

Another option, based on the proposed solution, that preserves the previous value or the new one chosen.

// Global variable to store current value
var oldValue = '';
// Blank value when click to activate all options
$('input').on('click', function() {
  oldValue = $(this).val();
  $(this).val('');
});
// Restore current value after click
$('input').on('mouseleave', function() {
  if ($(this).val() == '') {
    $(this).val(oldValue);
  }
});

https://jsfiddle.net/jpussacq/7Ldbc013/