>I'm sorry, but in my decade of JavaScript I never read anything more cryptic than this. What is this supposed to mean?

Switches in JS are just implicit maps.

  const caseVal = 'case1';

  const result = ({
   'case1': (()=> 'case1Val'),
   'case2': (()=> 'case2Val'),
   'case3': (()=> 'case3Val'),
  } [caseVal] || (()=> 'default'))()

Has identical performance to a switch case and is far more idiomatic.