Why Does JavaScript's `this.style[property]` Return an Empty String?

JavaScript's this.style[property] Returns an Empty String: Uncovering the Cause and Solution

When accessing the this.style[property] property in JavaScript, you might encounter situations where it retrieves an empty string instead of the expected value. This can be perplexing, especially if you're trying to retrieve style properties that have been set.

Understanding the Issue

this.style[property] is designed to access styles that have been directly assigned to the element in its HTML or via inline styles. If the requested property has not been explicitly set for the element, this.style[property] will return an empty string.

The Case of Cascading Styles

Often, styles for elements are defined in CSS stylesheets. These styles are cascaded, meaning that they are inherited and overridden based on specific rules. When an element matches multiple stylesheet rules, the most specific rule takes precedence.

In the example provided, the #test element has two styles defined in the stylesheet: height: 100px; and width: 100px;. However, the element also has an inline style of background: #CCC;, which overrides the #test stylesheet rule for background.

Due to the cascading nature of CSS, this.style[width] and this.style[height] will retrieve empty strings, as these properties have been set in the stylesheet but overridden by the inline style.

The Solution: getComputedStyle()

To retrieve the effective style property value, which includes the cascaded styles, you can use the getComputedStyle() method:

const myElement = document.getElementById('myId');
const style = getComputedStyle(myElement);
console.log(style.height); // "100px"

getComputedStyle() returns an object that represents the computed style of the specified element, taking into account all applied styles, including inherited and overridden properties.

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