flatten.js

flatten = array => {
  return array.reduce((sum, x) => {
    if (Array.isArray(x)) {
      return [...sum, ...flatten(x)];
    } else {
      return [...sum, x];
    }
  }, [])
};

assertEq = (a,b) => { 
  if (`${a}` !== `${b}`) { 
    throw `${a} !== ${b}`; 
  } 
};

assertEq( flatten([]) , [] );
assertEq( flatten([1]) , [1] );
assertEq( flatten([1,2]) , [1,2] );
assertEq( flatten([1,[2]]) , [1,2] );
assertEq( flatten([1,[2],[[],[],[]]]) , [1,2] );
assertEq( flatten([[1,2,[3]],4]) , [1,2,3,4] );