There are several ways to use javascript code

There are two ways to use JavaScript code, namely: 1. Direct execution. When a web page is opened, all JavaScript codes defined in script tags or linked js files will be executed; 2. Events Driver, when a certain event occurs, execute a certain piece of JavaScript code.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

[JavaScript execution method]

The JavaScript code defined in the HTML document has two execution methods: direct execution and event-driven.

Direct execution:

When we open a web page, all JavaScript code defined in the <script> tag or linked js file will will be executed. </script>

It should be noted that functions defined with function are not executed. The function will only be executed when it encounters a function call.

Example 1:

<script type="text/javascript">
var d = new Date();
var m = d.getMonth();
if( m>=5 )
document.write( m );
</script>

The above JavaScript code is placed naked in the <script> tag, and will be executed immediately when the web page is opened. </script>

Example 2:

<script type="text/javascript">
function getM()
{
var d = new Date();
var m = d.getMonth();
if( m>=5 )
document.write( m );
}
</script>
以上JavaScript代码定义的是函数，当网页打开时，函数中的代码不会被执行。当需要执行该函数时，需要使用函数调用。
<script type="text/javascript">
getM();
</script>

Event-driven:

When an event occurs, such as a web page being opened, a mouse click, a double-click, etc., Execute a piece of JavaScript code.

For example:

<script type="text/javascript">
var count = 0;
function setCount()
{
count++;
if( count>=5 )
count = 0;
}
</script>
<button onclick="setCount()">计数</button>

In this example, the tag defines a button, and the onclick attribute is used to respond to a mouse click event. When the button is clicked with the mouse , execute the setCount() function.

The attributes that respond to an event in the tag are called event handlers, and their values ​​are JavaScript codes.

Common event handles:

onload:

Triggered when the web page is opened. It is only valid within the

and tags.

This event is generally used to perform some initialization operations.

onunload:

Triggered when the web page is closed. It is only valid within the

and tags.

This event is generally used to complete some finishing work.

onclick:

Triggered when the mouse clicks. It can be used for objects such as controls, images, text, hyperlinks, etc.

This event is used to respond to mouse click operations and is the most commonly used event handler.

ondblclick:

Triggered when the mouse is double-clicked. It can be used for objects such as controls, images, text, hyperlinks, etc.

This event is used to respond to a mouse double-click operation.

onchange:

Triggered when the content changes. It can be used for objects such as text boxes, list boxes, etc.

This event is generally used to respond to the user's operation of modifying the content in the text box.

Description: When the user enters text into a text box, the onchange event will not be triggered. This event will only be triggered when the user clicks an area outside the text box after completing the input, causing the text box to lose focus. .

onselect:

Triggered when the content is selected. It can be used for objects such as text boxes, list boxes, etc.

This event is generally used to respond to operations such as the user selecting the content in the text box and changing the selected item in the list box.

The above are just a few of the most commonly used event handlers.

Note: The event handler is not a JavaScript code, but an HTML attribute, so it is not case-sensitive, but you should develop the habit of writing in lowercase letters.

Extended information:

[JavaScript definition method]:

There are two ways to add JavaScript code to HTML documents: embedded and linked. Mode.

Embedded:

Embed JavaScript code in the HTML document. Method:

<script type="text/javascript">
JS代码
</script>

JavaScript code must be defined between <script> and </script>.

<script> Tags can be placed in the head area or in the body area. </script>

In an HTML document, JavaScript code can appear in multiple places, and each place must be enclosed in <script> tags. </script>

Link-in:

Place the JavaScript code in a text file. The file extension should be defined as .js. Add the following tags to the HTML document:

<script type="text/javascript" src="js文件"></script>

The src attribute is used to specify the address of the linked js file. It can be a local file or a URL.

Using linking, the defined JavaScript code can be shared by multiple web pages.

[Recommended learning: javascript advanced tutorial]

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