JavaScript Where To

最新推荐文章于 2023-12-22 18:00:00 发布

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最新推荐文章于 2023-12-22 18:00:00 发布

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分类专栏： JavaScript 文章标签： javascript button extension date function html

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本文详细介绍了JavaScript如何在HTML页面中应用，包括在body和head部分的使用，通过实例展示了JavaScript函数和事件的使用方法，同时介绍了外部JavaScript文件的使用及在头部和主体中的放置策略。

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JavaScript Where To

JavaScripts can be put in the <body> and in the <head> sections of an HTML page.

JavaScript in <body>

The example below writes the current date into an existing <p> element when the page loads:

Example

<html>
<body><h1>My First Web Page</h1>
<p id="demo"></p>
<script type="text/javascript">
document.getElementById("demo").innerHTML=Date();
</script>
</body>
</html>

Try it yourself »

Note that the JavaScript is placed at the bottom of the page to make sure it is not executed before the <p> element is created.

JavaScript Functions and Events

JavaScripts in an HTML page will be executed when the page loads. This is not always what we want.

Sometimes we want to execute a JavaScript when an event occurs, such as when a user clicks a button. When this is the case we can put the script inside a function.

Events are normally used in combination with functions (like calling a function when an event occurs).

You will learn more about JavaScript functions and events in later chapters.

JavaScript in <head>

The example below calls a function when a button is clicked:

Example

<html><head>
<script type="text/javascript">
function displayDate()
{
document.getElementById("demo").innerHTML=Date();
}
</script>
</head>
<body>
<h1>My First Web Page</h1>
<p id="demo"></p>
<button type="button" onclick="displayDate()">Display Date</button>
</body>
</html>

Try it yourself »

Scripts in <head> and <body>

You can place an unlimited number of scripts in your document, and you can have scripts in both the body and the head section at the same time.

It is a common practice to put all functions in the head section, or at the bottom of the page. This way they are all in one place and do not interfere with page content.

Using an External JavaScript

JavaScript can also be placed in external files.

External JavaScript files often contain code to be used on several different web pages.

External JavaScript files have the file extension .js.

Note: External script cannot contain the <script></script> tags!

To use an external script, point to the .js file in the "src" attribute of the <script> tag:

Example

<html>
<head>
<script type="text/javascript" src="xxx.js"></script>
</head>
<body>
</body>
</html>

Try it yourself »

Note: Remember to place the script exactly where you normally would write the script!