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JavaScript 常用技巧：取值、鍵盤、視窗與捲動事件 ​

JavaScript 常用小技巧

這篇集中幾個 事件處理 上最常用到的小技巧，都可以直接複製下來測試。共同的套路都一樣：選取 DOM → 綁定事件 → 從 事件物件 取值 → 寫回畫面。

取得選單的值 ​

html
<div class="form">

  <select id="Select" class="form__select">
    <option value="" selected disabled>請選擇...</option>
    <option value="台北市">台北市</option>
    <option value="新北市">新北市</option>
    <option value="桃園市">桃園市</option>
    <option value="桃園縣">桃園縣</option>
    <option value="新竹市">新竹市</option>
    <option value="新竹縣">新竹縣</option>
    <option value="苗栗市">苗栗市</option>
    <option value="苗栗縣">苗栗縣</option>
  </select>

  <div class="form__msg" id="Box"></div>

</div>
js
document.querySelector('#Select').addEventListener('change', (e) => {
  document.querySelector('#Box').textContent = e.target.value;
});
css
.form {
  text-align: center;
}

.form__select {
  padding: .5rem 1rem;
  font-size: 1rem;
  border: solid 1px #aaa;
}

.form__msg {
  margin-top: 1rem;
}

小提醒

下拉選單用 change 就夠了，它在使用者選定後才觸發一次。寫入畫面用 textContent 最安全，相關差異請看 節點的屬性。

連續觸發取值 ​

html
<div class="form">
  <input type="text" id="InputText" placeholder="請輸入文字" class="form__input form__input--text">
  <div class="form__msg" id="Msg"></div>
</div>
js
document.querySelector('#InputText').addEventListener('input', (e) => {
  document.querySelector('#Msg').textContent = e.target.value;
});
css
.form {
  text-align: center;
}

.form__input {
  padding: .5rem 1rem;
  font-size: 1rem;
  border: solid 1px #aaa;
}

.form__msg {
  margin-top: 1rem;
}

input 每敲一個字就觸發一次，所以很適合做即時搜尋、字數統計這類需求；相對的，處理函式要盡量輕量。

鍵盤事件 ​

html
<div class="box">
  <h1>鍵盤事件</h1>
  <div class="box__msg" id="Msg"></div>
</div>
js
window.addEventListener('keyup', (e) => {
  console.log(e.key);
  document.querySelector('#Msg').textContent = e.key;
});
css
.box {
  text-align: center;
}

.box__msg {
  font-size: 2rem;
  font-family: Arial, Helvetica, sans-serif;
}

要判斷特定按鍵，直接比對 e.key 的名稱即可：

js
window.addEventListener('keyup', (e) => {
  if (e.key === 'Enter') {
    console.log('按了 Enter');
  }

  if (e.key === 'Escape') {
    console.log('按了 Esc，可以關閉彈窗');
  }
});

注意

舊教材常見的 e.keyCode 已被標記為過時，不同鍵盤配置的數值也不一致，請改用 e.key。

視窗大小變化 ​

html
<div class="box">
  <h1>可視範圍寬度</h1>
  <div class="box__msg" id="Msg"></div>
</div>
js
console.log(window.innerWidth);
console.log(window.innerHeight);

window.addEventListener('resize', () => {
  document.querySelector('#Msg').textContent = window.innerWidth;
});
css
.box {
  text-align: center;
}

.box__msg {
  font-size: 2rem;
  font-family: Arial, Helvetica, sans-serif;
}

小提醒

這裡刻意寫 window.innerWidth 而不是 this.innerWidth。箭頭函式沒有自己的 this，寫成 this 雖然在最外層剛好也是 window，但意圖不明確，詳見 this 是什麼。

捲動事件 ​

html
<article class="box">
  <h1>Lorem Ipsum</h1>
  <h2>What is Lorem Ipsum?</h2>
  <p>Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum.</p>
  <h2>Why do we use it?</h2>
  <p>It is a long established fact that a reader will be distracted by the readable content of a page when looking at its layout. The point of using Lorem Ipsum is that it has a more-or-less normal distribution of letters, as opposed to using 'Content here, content here', making it look like readable English. Many desktop publishing packages and web page editors now use Lorem Ipsum as their default model text, and a search for 'lorem ipsum' will uncover many web sites still in their infancy. Various versions have evolved over the years, sometimes by accident, sometimes on purpose (injected humour and the like).</p>
  <h2>Where does it come from?</h2>
  <p>Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old. Richard McClintock, a Latin professor at Hampden-Sydney College in Virginia, looked up one of the more obscure Latin words, consectetur, from a Lorem Ipsum passage, and going through the cites of the word in classical literature, discovered the undoubtable source. Lorem Ipsum comes from sections 1.10.32 and 1.10.33 of "de Finibus Bonorum et Malorum" (The Extremes of Good and Evil) by Cicero, written in 45 BC. This book is a treatise on the theory of ethics, very popular during the Renaissance. The first line of Lorem Ipsum, "Lorem ipsum dolor sit amet..", comes from a line in section 1.10.32.</p>
  <p>The standard chunk of Lorem Ipsum used since the 1500s is reproduced below for those interested. Sections 1.10.32 and 1.10.33 from "de Finibus Bonorum et Malorum" by Cicero are also reproduced in their exact original form, accompanied by English versions from the 1914 translation by H. Rackham.</p>
  <h2>Where can I get some?</h2>
  <p>There are many variations of passages of Lorem Ipsum available, but the majority have suffered alteration in some form, by injected humour, or randomised words which don't look even slightly believable. If you are going to use a passage of Lorem Ipsum, you need to be sure there isn't anything embarrassing hidden in the middle of text. All the Lorem Ipsum generators on the Internet tend to repeat predefined chunks as necessary, making this the first true generator on the Internet. It uses a dictionary of over 200 Latin words, combined with a handful of model sentence structures, to generate Lorem Ipsum which looks reasonable. The generated Lorem Ipsum is therefore always free from repetition, injected humour, or non-characteristic words etc.</p>
</article>
js
window.addEventListener('scroll', () => {
  console.log(window.scrollY);
});
css
.box {
  max-width: 500px;
  margin-left: auto;
  margin-right: auto;
}

window.scrollY 是垂直捲動距離，水平則是 window.scrollX。實務上常拿它做回到頂端按鈕、捲到某個位置才出現的動畫。

效能注意

scroll 與 resize 都是高頻事件，短時間內可觸發上百次。處理函式若要做較重的計算，請搭配 定時器 做節流（throttle）或去彈跳（debounce）。

將選取起來的 DOM 指定到變數中 ​

  • 變數使用可簡化程式碼的重複撰寫。
  • 變數會儲存於記憶體當中，當在重複使用時，會直接到記憶體中進行取用，不必每次都從文件中重複查詢。
html
<div class="box" id="Box">
  <h1 class="box-title">title</h1>
</div>
js
// const elemTitle = document.querySelector('.box-title');
// const elemTitle = document.querySelector('#Box .box-title');

const elemBox = document.querySelector('#Box');
// const elemTitle = elemBox.querySelector('.box-title');
// const elemTitle = elemBox.children[0];
// const elemTitle = elemBox.firstElementChild;
const elemTitle = elemBox.getElementsByClassName('box-title')[0];

小提醒

上面幾種寫法都能選到同一個元素，差別只在可讀性與範圍。實務上多用 elemBox.querySelector('.box-title')：從父層往下找，範圍明確，也不怕頁面其他地方有同名 class。

常見問題 ​

要取得表單的值，用 change 還是 input？

下拉選單用 change，選定後觸發一次即可；文字輸入要即時反應用 input，每次值變動都會觸發。兩者都能用 e.target.value 取值。

keyCode 為什麼不建議再用？

keyCode 已被標記為過時，且不同鍵盤配置的數值不一致。改用 e.key 取得按鍵名稱，例如：Enter、Escape、a，語意清楚也更可靠。

怎麼取得目前的視窗寬高？

用 window.innerWidth 與 window.innerHeight，它們是含捲軸的可視範圍尺寸。搭配 resize 事件，就能在視窗變動時同步更新。

捲動位置要怎麼取得？

用 window.scrollY 取得垂直捲動距離，水平則是 window.scrollX。監聽 window 的 scroll 事件即可持續得到最新值。

為什麼要把選取到的 DOM 存進變數？

每次呼叫選取方法都要重新查詢整份文件，存進變數後即可直接從記憶體取用，重複使用時效能較好，程式碼也更簡潔好讀。

延伸閱讀 ​

您了解事件物件嗎？ 代表圖您了解事件物件嗎？事件委派 代表圖事件委派節點的屬性 代表圖節點的屬性定時與排程 代表圖定時與排程

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