Correct way to animate box-shadow with jQuery

Which is the correct syntax to animate the box-shadow property with jQuery?

$().animate({?:"0 0 5px #666"});

Direct answer

Using Edwin Martin's jQuery plugin for shadow animation, which extends the .animate method, you can simply use the normal syntax with "boxShadow" and every facet of that - color, the x- and y-offset, the blur-radius and spread-radius - gets animated. It includes multiple shadow support.

$(element).animate({ 
    boxShadow: "0px 0px 5px 3px hsla(100, 70%, 60%, 0.8)"
}) 

Using CSS animations instead

jQuery animates by changing the style property of DOM elements, which can cause surprises with specificity and moves style information out of your stylesheets.

I can't find browser support stats for CSS shadow animation, but you might consider using JS to apply an animation-based class instead of handling the animation directly. For example, you can define a box-shadow animation in your stylesheet:

@keyframes shadowPulse {
    0% {
        box-shadow: 0px 0px 10px 0px hsla(0, 0%, 0%, 1);
    }

    100% {
        box-shadow: 0px 0px 5px 0px hsla(0, 0%, 0%, 0);
    }
}
    
.shadow-pulse {
    animation-name: shadowPulse;
    animation-duration: 1.5s;
    animation-iteration-count: 1;
    animation-timing-function: linear;
}

You can then use the native animationend event to synchronise the end of the animation with what you were doing in your JS code:

Vanilla JS:

element.classList.add('shadow-pulse')
element.addEventListener('animationend', event => {  
    element.classList.remove('shadow-pulse')
    // do something else...
})

jQuery:

$(element).addClass('shadow-pulse')
$(element).on('animationend', function(){    
    $(element).removeClass('shadow-pulse')
    // do something else...
})