placeholderFromDatalist.js

(function () {
  'use strict';

  var doc = this.document,
      i, inputs, input, options, rand;

  if (!doc.querySelectorAll) return;

  inputs = doc.querySelectorAll('input[list]');

  for (i = inputs.length - 1; i >= 0; i--) {
    input   = inputs[i];
    options = doc.getElementById(input.getAttribute('list')).getElementsByTagName('option');
    rand    = Math.floor(Math.random() * options.length);

    input.setAttribute('placeholder', options[rand].getAttribute('value'));
  }
}).call(this);