Why Doesn't JavaScript Mouseover Always Trigger CSS :hover?

Simulating Mouseover and Triggering CSS ":hover" Declaration

Problem Introduction

Some developers have encountered an issue where trying to simulate mouseover functionality in pure JavaScript does not trigger the CSS hover declaration, despite triggering the mouseover listener. This article seeks to address this challenge and explore potential solutions.

Explanation of Event Trust Levels

At the core of the problem lies the concept of event trust levels in the browser. Events that are triggered by user interaction or changes to the DOM are considered trusted events and have certain privileges. In contrast, events generated by JavaScript code through the DocumentEvent.createEvent or EventTarget.dispatchEvent methods are considered untrusted events.

Restrictions on Untrusted Events

The lack of triggering "hover" styles for untrusted events is due to security restrictions. Browsers limit the default actions triggered by untrusted events, except for those like click or DOMActivate. This is a deliberate measure to prevent malicious code from simulating user interactions and potentially compromising security.

Solution: Adding CSS Classes Manually

To simulate mouseover effects, the solution lies in manually adding and removing CSS classes when mouseover and mouseout events are detected. This approach effectively triggers the CSS ":hover" declaration without relying on untrusted events. Here's an example implementation:

// Simulate mouseover effect
element.addEventListener("mouseover", () => {
  element.classList.add("hover");
});

// Simulate mouseout effect
element.addEventListener("mouseout", () => {
  element.classList.remove("hover");
});

By implementing this solution, you can simulate the mouseover event and trigger the desired CSS hover styles without violating browser security restrictions.

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