JavaScript 代码块中的“选项”来自哪里？

Question

i have this code from MDN which i am trying to understand !

第一：>

“options”在这个块中来自哪里:-

for (var i = 0; i < selectObject.options.length; i++) { if (selectObject.options[i].selected)*

秒：>

  why it 'options' does work fine inside the function and not work outside the function

这是代码-

<form name="selectForm">
  <p>
    <label for="musicTypes">Choose some music types, then click the button below:</label>
    <select id="musicTypes" name="musicTypes" multiple="multiple">
      <option selected="selected">R&B</option>
      <option>Jazz</option>
      <option>Blues</option>
      <option>New Age</option>
      <option>Classical</option>
      <option>Opera</option>
    </select>
  </p>
  <p><input id="btn" type="button" value="How many are selected?" /></p>
</form>

<script>
function howMany(selectObject) {
  var numberSelected = 0;
  for (var i = 0; i < selectObject.options.length; i++) {
    if (selectObject.options[i].selected) {
      numberSelected++;
    }
  }
  return numberSelected;
}

var btn = document.getElementById('btn');
btn.addEventListener('click', function() {
  alert('Number of options selected: ' + howMany(document.selectForm.musicTypes));
});
</script>

Answer 1

1) options 是 HTML DOM 的一部分，负责 selecting options 指定 select 元素内的元素

2) 它确实在任何 select 元素的函数之外工作，但我假设您正在尝试在函数之外使用 selectObject，这违反了变量范围。

阅读更多：

[1] https://www.w3schools.com/jsref/coll_select_options.asp

[2]https://www.w3schools.com/js/js_scope.asp

JavaScript 代码块中的“选项”来自哪里？

where does ''options" come from in this JavaScript block of code?

javascript

for-loop

html-select

function