Web scrapers of the CSS and jQuery variety refer to the code used to interact with certain elements in the Document Object Model. These codes (or functions) can be very useful when you need to act or extract specific elements of the page you are trying to scrape without changing its other elements. A smart use of automated web scraping software and the right selectors will help you manipulate the target page as you planned or isolate the data you want to scrape.
However, a good number of people are very familiar with the way the internet works. A lesser number know how a web document is arranged and the hierarchy concept for HTML as represented in the Document Object Model. Since DOM will be the basis for the choice of CSS and jQuery selectors, here is a brief look at the idea of the DOM.
The Document Object Model
When you click on a webpage and it opens, the browser gets you the page HTML document. However, that’s just a representation (in text form) of the document’s object tree. Browsers cannot use this data this way; they have to analyze it as an internal tree structure. Only after doing this can they begin working on it, as you see on the page displayed in your browser. The structure the browser uses is termed the Document Object Model.
What you will find on the usual demo page is always different from what you will get when you open a browser’s developer tools (for Chrome and Firefox, the shortcut is F12). You can also access it by clicking “Elements” in Chrome and “Inspector” in Firefox. That is where you will find the DOM structure the browser in use is using at the time to render the web page you opened.
Note that if a website uses JavaScript to change its content, the code can alter the DOM tree at any time and might be rendered differently from the initially used HTML code, which users can check by opening the “View Page Source” option.
Understanding the hierarchical concept of the DOM is vital, as it is the concept on which CSS and jQuery selectors are based.
What Is A CSS Selector?
Ever heard of Cascading Style Sheets? This is where CSS originates from.
Their initial aim was to be the foremost aspect of the CSS rule, even ahead of the declaration block (this defined what document the rule would be applied to). Here is an example.
a {
color: red;
}
CSS is a powerful system primarily used by web designers to keep visual aspects such as fonts and colors) separate from the general document structure.
One reason the use of CSS was extended beyond CSS itself was simply that the selector concept offers a straightforward approach to identifying random elements in any HTML document. CSS selectors are a significant part of JavaScript frameworks and JavaScript frontend development.
However, CSS selectors are not JavaScript-limited. Most languages support them in different ways – either through third-party libraries or natively.
This is where it becomes useful in data extraction or, more specifically, scraping, where locating data is a significant aspect. CSS selectors are a crucial tool that can help make your codes better and appear more refined and elegant. Do you know what else makes codes appear smart? Continue reading to find out.
What Are jQuery Selectors?
jQuery selectors identify and change HTML elements, just like CSS selectors. They are a significant part of the jQuery library.
Again, like CSS selectors, you can locate HTML elements that correspond to the element’s class, types, id, and attributes on a DOM. jQuery selectors are based on existing CSS selectors and have their selectors also.
One difference between CSS and jQuery selectors is that all jQuery selectors begin with the dollar sign and the parenthesis {$()} in their use. This is termed the factory function.
Examples of Some CSS and jQuery Selectors
Here are some CSS and jQuery selectors:
Basic Selectors
ID Selector
This selector highlights a specific element with a unique identifier. You can identify the element using the id attribute value. For example, if you want to select the element with the ID “main,” you can use the #main selector.
Element Selector
This selector highlights all elements with a selected element name. For example, if you want to select all the paragraph elements in a document, you can use the p selector.
Class Selector
This selector highlights all elements that share a specific class. You can identify the class using the class attribute value. For example, if you want to select all the elements with the class “container,” you can use the .container selector.
Some Advanced Selectors
Attribute Selector
This selector highlights an element that has a specific attribute. For example, if you want to select all the input elements with the type “text,” you can use the input[type= “text”] selector.
Child Selector
This selector highlights an element that is a direct child of another element. For example, if you want to select all the span elements that are direct children of a div element, you can use the div > span selector.
Descendant Selector
This selector highlights an element that is a descendant of another element. For example, if you want to select all the span elements that are descendants of a div element, you can use the div span selector.
Pseudo-class Selector
This selector highlights an element based on its state or position.
Note that CSS and jQuery selectors work the same way, except in the instances of jQuery selectors that use the dollar sign and the parentheses, as previously stated.
Conclusion
CSS and jQuery selectors can be highly useful in web scraping. The examples listed in this article are the most basic and commonly used ones; when you continue your journey in web scraping, you will find out more. Knowing the functions and capabilities of each selector type can improve your flexibility and help you get what you need as fast as possible. Good luck finding other selector examples.