flatten-array.js

//JS (es6)
//
const crazyArray = [[1,2,3], [1,2,3,4,5,[6, [7, [8]]]], [1,1,1,1,1,1,1,1]];

flatten = (arr) => {
  if(!Array.isArray(arr)){
    throw "Parameter is not an array!";
  }
  
  return arr.reduce((acc, el) => {
      if(Array.isArray(el)) {
        return [...acc, ...flatten(el)];
      }
      return [...acc, el];
  }, [])
};