When creating an iife, use a function, NOT a fat arrow. With a fat arrow, this is bound to the this of the surrounding code (in this case Window or Global), negating the use of an iife (to prevent polluting global scope).

arrow_vs_function_iife.js

// When creating an iife, use a function, NOT a fat arrow.
// With a fat arrow, this is bound to the this of the surrounding code (in this case Window or Global).
// Therefore, using a fat arrow negates the use of an iife (to prevent polluting global scope).

(() => {
  'use strict';
  
	console.log('fat arrow', this)
})();

(function(){
  'use strict';

  console.log('function', this)
})();