Question

jQuery 1.4.2’s animate() API spec is

.animate( properties, [ duration ], [ easing ], [ callback ] )

but it seems that we can supply duration, callback, and no easing

.animate({left: '+= 100'}, 600, doThis)

and it will work.

But if we supply easing and callback and no duration

.animate({left: '+=100'}, 'swing', doThis)

then the easing won’t be taken into effect. So what exactly is the API supposed to be?

Update: please see my answer below.

Answer 1

Normally jQuery determines optional parameters by types, e.g. this works:

$('.class').animate({opacity: 'toggle'}, doThis);

But in the case of duration/easing, since they’re both strings it can’t (and if there’s a single string, it assumes it’s the duration). You need to either call it with 'normal' as the duration, like this:

.animate({left: '+=100'}, 'normal', 'swing', doThis)

Or use the .animate(options, properties) version, like this:

.animate({left: '+=100'}, { easing: 'swing', complete: doThis })