How to call JavaScript programs on web pages

JavaScript is a scripting language widely used in Web development. It can realize dynamic effects, interactive experience, form verification, asynchronous loading and data processing and other functions. It is a modern An integral part of web development. During the development process, JavaScript code needs to be embedded into HTML files and specific methods are used to call these programs to achieve the required functions. This article will introduce several common ways to call JavaScript programs, and explain their principles and applicable scenarios.

1. Inline script

Inline script is a way to embed JavaScript code directly into an HTML document by using the script tag. For example:

    
  
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In this example, we embed an inline script in the body tag of the HTML file, which will output a message to the console when the page loads. The advantage of inline script is that it is simple, fast and intuitive. It does not require additional HTTP requests and file loading, and can be executed directly in the browser. However, its maintainability is poor and it is not easy to reuse, test and modify. For more complex programs, inline scripts are not recommended.

2. External script

External script is a way to save JavaScript code in a separate .js file and then introduce it through the HTML script tag. For example:

     
  
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In this example, we introduced an external script file script.js in the head tag of the HTML file, which contains a program that outputs Hello World. The advantage of external scripts is that they have good maintainability and reusability. JavaScript code can be written, tested and modified independently, and can be shared between multiple pages. However, it requires additional HTTP requests and file loading time, increasing the response time of the web page.

3. Event Handler

Event handler is a way to associate JavaScript code with events of HTML elements, such as click events, mouse movement events, keyboard Key events, etc. For example:

    
  
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In this example, we add an onclick event handler in the button tag of the HTML file. When the user clicks the button, a message box will pop up to display Hello World. The advantage of the event handler is that JavaScript code can be specified directly in the HTML page without the need for additional file loading and tag introduction. However, its maintainability is poor and it is not easy to reuse, test and modify. For more complex programs, it cannot It is recommended to use event handlers.

4. External link

External link is a way to save JavaScript code in an external file and use thetag to reference it. For example:

     
  
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In this example, we use an external link to introduce the script.js file on the remote server into the current HTML file. This file contains a code that outputs Hello World. The advantage of external links is that multiple external files can be introduced to separate hard coding and soft coding. At the same time, technologies such as CDN can also be used to optimize loading speed and security issues. However, you need to pay attention to the reliability, performance and cross-domain issues of external links to ensure that the code can run normally and protect user privacy.