Trigger CSS transition on appended element

Please use the below code, use "focus()"

Jquery

var $a = $('<div>')
    .addClass('box a')
    .appendTo('#wrapper');
$a.focus(); // focus Added
$a.addClass('in');

Javascript

var e = document.createElement('div');
e.className = 'box e';
document.getElementById('wrapper').appendChild(e).focus(); // focus Added
e.className += ' in';

Using jQuery try this (An Example Here.):

var $a = $('<div>')
.addClass('box a')
.appendTo('#wrapper');
$a.css('opacity'); // added
$a.addClass('in');

Using Vanilla javaScript try this:

var e = document.createElement('div');
e.className = 'box e';
document.getElementById('wrapper').appendChild(e);
window.getComputedStyle(e).opacity; // added
e.className += ' in';

Brief idea:

The getComputedStyle() flushes all pending style changes and forces the layout engine to compute the element's current state, hence .css() works similar way.

About css()from jQuery site:

The .css() method is a convenient way to get a style property from the first matched element, especially in light of the different ways browsers access most of those properties (the getComputedStyle() method in standards-based browsers versus the currentStyle and runtimeStyle properties in Internet Explorer) and the different terms browsers use for certain properties.

You may use getComputedStyle()/css() instead of setTimeout. Also you may read this article for some details information and examples.

@Frizi's solution works, but at times I've found that getComputedStyle has not worked when I change certain properties on an element. If that doesn't work, you can try getBoundingClientRect() as follows, which I've found to be bulletproof:

Let's assume we have an element el, on which we want to transition opacity, but el is display:none; opacity: 0:

el.style.display = 'block';
el.style.transition = 'opacity .5s linear';

// reflow
el.getBoundingClientRect();

// it transitions!
el.style.opacity = 1;

The cause of not animating the newly added element is batching reflows by browsers.

When element is added, reflow is needed. The same applies to adding the class. However when you do both in single javascript round, browser takes its chance to optimize out the first one. In that case, there is only single (initial and final at the same time) style value, so no transition is going to happen.

The setTimeout trick works, because it delays the class addition to another javascript round, so there are two values present to the rendering engine, that needs to be calculated, as there is point in time, when the first one is presented to the user.

There is another exception of the batching rule. Browser need to calculate the immediate value, if you are trying to access it. One of these values is offsetWidth. When you are accessing it, the reflow is triggered. Another one is done separately during the actual display. Again, we have two different style values, so we can interpolate them in time.

This is really one of very few occasion, when this behaviour is desirable. Most of the time accessing the reflow-causing properties in between DOM modifications can cause serious slowdown.

The preferred solution may vary from person to person, but for me, the access of offsetWidth (or getComputedStyle()) is the best. There are cases, when setTimeout is fired without styles recalculation in between. This is rare case, mostly on loaded sites, but it happens. Then you won't get your animation. By accessing any calculated style, you are forcing the browser to actually calculate it.