<input type="number" id="number-field">

<p>値の型は...「<span id="type"></span>」</p>

var field = document.getElementById("number-field");
var typeName = document.getElementById("type");

var setType = function(val) {
  typeName.innerText = typeof val;
}

field.addEventListener("input", function(e) {
  setType(e.target.value);
})

window.onload = function() {
  field.value = 20; // 最初に数値を入れても...
  setType(field.value);
}