jQuery - jQuery’s .fadeIn() and .fadeOut() Internals

The .fadeIn() and .fadeOut() methods in jQuery are animation utilities used to show or hide elements by gradually changing their opacity. Instead of instantly appearing or disappearing, elements transition smoothly, improving visual flow and user experience.

Core Mechanism Behind Fading
Internally, jQuery manipulates the CSS opacity property of an element. For .fadeIn(), opacity is increased step by step from 0 to 1. For .fadeOut(), opacity is decreased from 1 to 0. These changes happen over a defined duration using a timed loop.

Use of the Effects Queue
Both methods rely on jQuery’s effects queue (fx). When a fade animation is triggered, it is added to the queue and runs only after previous animations on the same element finish. This prevents visual conflicts and ensures smooth, sequential animations.

Display Property Handling
Before a fade-in begins, jQuery sets the element’s display property to a visible state if it was hidden. After a fade-out completes, jQuery sets display: none to fully hide the element. This ensures the element does not occupy layout space when invisible.

Timing and Easing Control
jQuery uses timers to calculate small opacity changes over time. Easing functions determine how fast the opacity changes at different points in the animation. This is why fades feel smooth instead of abrupt, even though they are made of many small steps.

Why These Internals Matter
Understanding how .fadeIn() and .fadeOut() work helps developers avoid animation stacking, unexpected visibility issues and performance problems. It also explains why rapid triggering of fade effects can cause delays if the queue is not managed properly.