JavaScript definition method and JavaScript execution method

JavaScript definition method:

There are two methods for adding JavaScript code to HTML documents: embedded and linked. .

Embedded:

Embed JavaScript code in the HTML document. Method:

JavaScript code must be defined Between .

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.

JavaScript execution method

There are two execution methods for JavaScript code defined in HTML documents: direct execution and event-driven.

Direct execution:

When we open a web page, all JavaScript code defined in the

The above JavaScript code is placed naked in the

The above JavaScript code defines a function. When the web page is opened, the code in the function will not be executed. When you need to execute this function, you need to use a function call.

Event driven: < /p>

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

For example:

Count

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

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

Common event handles:

onload:

Triggered when the web page is opened. It only works within and tags.

This event is generally used to perform some initialization operations.

onunload:

Triggered when the web page is closed. It only works within 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 modification of the content in the text box.

Note: When the user enters text into a text box, the onchange event will not be triggered. It 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. event.

onselect:

Triggered when 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 using lowercase letters.

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