我正在使用這個(gè)W3C Schools 自動(dòng)完成下拉菜單。function autocomplete(inp, arr) {  /*the autocomplete function takes two arguments,  the text field element and an array of possible autocompleted values:*/  var currentFocus;  /*execute a function when someone writes in the text field:*/  inp.addEventListener("input", function(e) {      var a, b, i, val = this.value;      /*close any already open lists of autocompleted values*/      closeAllLists();      if (!val) { return false;}      currentFocus = -1;      /*create a DIV element that will contain the items (values):*/      a = document.createElement("DIV");      a.setAttribute("id", this.id + "autocomplete-list");      a.setAttribute("class", "autocomplete-items");      /*append the DIV element as a child of the autocomplete container:*/      this.parentNode.appendChild(a);      /*for each item in the array...*/      for (i = 0; i < arr.length; i++) {        /*check if the item starts with the same letters as the text field value:*/        if (arr[i].substr(0, val.length).toUpperCase() == val.toUpperCase()) {          /*create a DIV element for each matching element:*/          b = document.createElement("DIV");          /*make the matching letters bold:*/          b.innerHTML = "<strong>" + arr[i].substr(0, val.length) + "</strong>";          b.innerHTML += arr[i].substr(val.length);          /*insert a input field that will hold the current array item's value:*/          b.innerHTML += "<input type='hidden' value='" + arr[i] + "'>";          });          a.appendChild(b);        }      }  });我對(duì)這個(gè)自動(dòng)完成功能中的這行代碼感到困惑：inp.value = this.getElementsByTagName("input")[0].value;this上面的代碼指的是什么？我們?yōu)槭裁匆@樣做this.getElementsByTagName("input")[0]？我正進(jìn)入（狀態(tài)：未捕獲的類型錯(cuò)誤：無法讀取未定義的屬性“值”在這一行。然而，我確實(shí)修改了上面的函數(shù)并刪除了inp參數(shù)上的添加事件偵聽器行。