flatten.js

function flatten() {
  var result = [];
  var currArg;
  for(var i = 0; i < arguments.length; i++) {
    currArg = arguments[i];
    if (currArg.constructor === Array) {
      result.push.apply(result, flatten.apply(this, currArg))
    } else {
      result.push(currArg);
    }
  }
  return result;
}

console.log('output ', flatten([
  1, 2, 3, 4, 5, 6, [7, 8, 9, [0, ['a', 'b', 'c']]]
]))

// output  [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 'a', 'b', 'c' ]