HTML query function: understand selectors and query methods in HTML-Front-end Q&A-php.cn

HTML (Hypertext Markup Language) is one of the most commonly used languages in web development and is used to build web pages and web applications. HTML defines the structure and style of elements on the page, and when building the page, developers need to query and operate on the elements. Therefore, HTML provides a very convenient query method to locate and operate elements - selectors.

A selector is a method of querying elements. It can locate one or more elements in an HTML page and operate on these elements. On this basis, developers can build their own selectors to implement complex element queries and operations.

The syntax and usage of HTML selectors are similar to CSS (Cascading Style Sheets), and the relationship between them is very close. In HTML, there are three basic types of selectors:

1. Tag selector
The tag selector is one of the most basic selectors in HTML, that is, to query the corresponding HTML element according to its tag. element. Its syntax is very simple, just use the element name as the selector. For example, use the "

" tag to select all

elements on the HTML page:

var div_elements = document.querySelectorAll("div");

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2.ID selector
ID is the unique identifier in the HTML element, ID selection The processor queries the corresponding element based on its ID attribute. Its syntax requires adding the "#" symbol in front of the selector, for example:

var element = document.querySelector("#myId");

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3. Class selector
The class selector queries the corresponding element according to the class attribute of the HTML element. The syntax of a class selector is very similar to that of an ID selector, but a "." symbol is added in front of the selector, for example:

var elements = document.querySelectorAll(".myClass");

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In addition to basic selectors, HTML also provides a series of advanced selectors , such as attribute selectors, pseudo-class selectors, etc., can help developers locate and operate elements more accurately.

Finding HTML elements: using query methods

In addition to using selectors, HTML also provides some query methods for finding certain types of HTML elements. These query methods can well complement the functions of selectors, allowing developers to use HTML elements more flexibly.

1.getElementById()
getElementById() is a very practical query method in HTML. It can return the corresponding element based on the ID attribute of the HTML element. For example:

var element = document.getElementById("myId");

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2.getElementsByTagName()
getElementsByTagName() can be used to query all elements of an HTML element tag. For example, if you want to find all elements of all

tags in an HTML page, you can use the following code:

var h1_elements = document.getElementsByTagName("h1");

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3.getElementsByClassName()
getElementsByClassName() is a query method that can be used Search for elements based on their class names. For example:

var elements = document.getElementsByClassName("myClass");

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4.querySelector() and querySelectorAll()
querySelector() and querySelectorAll() are two very practical query methods that can help you use CSS selectors to query matching HTML elements. For example:

var element = document.querySelector("div");
var elements = document.querySelectorAll(".myClass");

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Summary:

HTML selectors and query methods are basic skills for web developers. Mastering them will make it easier to write HTML programs and improve development efficiency. Understanding the principles and syntax of selectors and query methods in HTML can allow us to better locate and operate HTML elements, making it faster and more efficient when building web applications.

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