var btn = document.getElementById('replace');

var i = 0;

​

var replaceInputs = function replaceInputs() {

  var inputs = {

    user: 'text',

    password: 'password'

  };

  i++;

  Object.keys(inputs).forEach(function(id) {

    var el = document.getElementById(id);

    var div = document.getElementsByClassName(id)[0];

    div.removeChild(el);

    el = document.createElement('input');

    el.name = id;

    el.placeholder = id + i;

    el.id = id;

    el.type = inputs[id];

    div.appendChild(el);

    div = el = null;

  });

};

​

btn.addEventListener('click', replaceInputs, false);