I’ve noticed this video while ago, and I wonder how it is possible to preview current node?
I tried everything but I’m done with new ideas 
@edit
Shift + H doesn’t work.
I guess you are asking about eevee, but it should work similarly like in cycles.
Just switch to material view or rendered view. Shift-Z
I don’t get it. Shift + Z is wireframe mode.
I see on video that it is in EEVEE mode and suddenly clicks on node, and it previews it. How?
Shift-z switches between modes. Click it a couple of times.
You can preview certain nodes with Node Wrangler, you have to enable it first and scroll through the settings, i think the shortcut was Ctrl Shift LMB but not really sure.
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Dude, thanks!!!
I really love hidden features in that software … ehh. Blender is really great at this part 
Oh viewer node. Here’s more ‘tricks’ with node wrangler - it’s manual. 
https://gregzaal.github.io/node-wrangler/
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Such preview only works for Cycles, not Eevee…