[SalesForce] Polling in lightning component

The empty arguments is a bit strange - a side effect of the way your renderer method is invoked by Lightning (Aura Framework) - but if you console.log(cmp) and console.log(helper) you will see that they are in scope and available.

You do not have to use a Helper at all unless you have common code you want to be able to call from other renderer (or controller) methods.

Implementing render(), rerender(), etc allows you to take over the DOM generation and update process entirely or (what I do more often) is to let Lightning markup do some of the work and then custom javascript (often 3rd party libs) fill in the blanks.

Even better in many cases is to delegate all of the rendering work to Lightning and leverage its powerful data binding {!v.something} capabilities with auto update (even bidrectional if you need that). So instead of creating a renderer you can have your .cmp use {!v.something} expressions and just have your client side controller update the underlying model/attribute values and Lightning will automatically (and very efficiently) rerender the subregions from your cmp that are impacted.

I often combine all of the above to achieve the best balance.

(The post below actually doenst work. CSV will cause a 404, because that file type never generates a preview. But PDF/xlxs/doc etc will show a broken link, but not a 404... )

For my purpose, the onerror event worked. There are many examples on stack exchange.

<img  onerror="{!c.imageError}" src="{!'/sfc/servlet.shepherd/version/renditionDownload?rendition=THUMB720BY480&amp;versionId='+ v.LatestPublishedVersionId }"

If there is a '404 image not found' error, the event fires, and then I hide the div containing the Image, and unhide a div with a message to the effect of 'Your image preview will be available shortly'. Then the next time the user comes back, or refreshes the page, the image should appear.

I figure I could do some kind of polling or reloading the image every second to check if the 404 Error still exists, but Im worried that may lead to other browser issues. I may look for a better solution later, but this User Experience is more acceptable than a broken image.