Flatten with Generators

flatten.js

export default function flatten(arrayOfArrays=[]){
  function* flatgen() {
    for( let item of arrayOfArrays ) {
      if ( Array.isArray( item )) {
        yield* flatten(item)
      } else {
        yield item
      }
    }
  }
  
  return [...flatgen()];
}

var flatArray = flatten([[1, [2]], [3], [4, 5], [[6, 7], 8], 9]);

console.dir( flatArray );