JS 語法的放置位置

1. Intro

In HTML, JavaScript statements are "instructions" to be "executed" by the web browser.

2. In HTML

  • Must be inserted between <script> and </script> tags.
  • <script type="text/javascript">: This type attribute is not required.
    • (JavaScript is the default scripting language in HTML).
    • EX:
<script>
function myFunc() {
    alert('Hi');
}
</script>
<button type="button" onclick="myFunc()">Try it</button>

  • JavaScript 放在 HTML 中的 <body> 或 <head> sections 裡都行，沒有個數限制

    • 即使放在 <html> 之外也能 work (on Chrome)

  • Keeping all code in one place, is always a good habit.

    • It is a good idea to place scripts at the bottom of the <body> element. This can improve page load, because script compilation can slow down the display.
    • You can place an external script reference in <head> or <body> as you like. The script will behave as if it was located exactly where the <script> tag is located.

3. External JavaScript (外部的 JS 檔)

  • 引用語法：
<script src="myScript.js"></script>
  • External scripts (e.g. "myScript.js") 裡直接就是JS語法，不用也不能再有<script>標籤(因為這是HTML標籤)
  • 外部JS檔的優點
    1. It separates HTML and code
    2. It makes HTML and JavaScript easier to read and maintain
    3. Cached JavaScript files can speed up page loads

簡單來說就是：更易維護+有助於JS快取來加強頁面讀取速度

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