Map left mouse button in Karabiner Elements I want to map CapsLock to Left Mouse Button using Karabiner Elements in macOS Sierra. 

I used to have such mapping in previous OS X versions using Seil and Karabiner but those no longer work in Sierra.

I tried to guess what should I put in the configuration JSON but failed. 

"simple_modifications" : {
    "caps_lock" : WHAT_SHOULD_I_PUT_HERE?
},


Is it even possible to map Caps Lock to Left Mouse button in Karabiner Elements?
BUT: it's actually quite simple (if I didn't mis-interpret your question):

Just select [Add item] in K.-Element's "Simple Modifications" tab, choose "caps_lock" (left) and "button1" (right) – and you're DONE!

… no need for "Complex Modifications", btw.