[Achshar]

Wasn't this possible already? Put a breakpoint or 'breakpoint;' on the function body's first line and it will pause execution when the function is called synchronously or asynchronously. And browse the call stack on the right column. What's new here? I have been doing this for quite sometime.

[paulirish]

Asynchronous hops have never been traceable in JavaScript callstacks before. This feature allows you to hop across the asynchronous gap of a bunch of APIs: addEventListener, setInterval, setTimeout, XMLHttpRequest, promises, requestAnimationFrame, and MutationObservers.

More visually, check out the Before/After images from the doc: http://imgur.com/rZlgrMe

[EricDeb]

The difference (I believe) is in the stack trace depth, specifically that the trace doesn't die if it happens to go through an async event. You can see in the first gif that if the async box is checked it allows a developer to see the stack trace all the way back to when the form submit event was initiated, despite the fact that an async ajax call was made in the middle of the stack. Normally, the stack trace stops at a response to an async event. You can see this in the beginning of the gif without the async box checked.

[acdanger]

This assumes you know the function that's being triggered. It's not always clear. And throw JQuery into the mix and things get murkier pretty quickly.

[Cthulhu_]

A means to group entries in a call stack together would be neat too; in most cases, something you're tracing won't be an issue in jquery or your library, so collapsing those would be neat.