.flat() is not a function, what's wrong?

Question

The following code

function steamrollArray(arr) {
  // I\'m a steamroller, baby
  return arr.flat();
}

steamrollArray([1, [2], [3, [[4]]]]);

Answer 1

Not sure if it is a valid answer however in my attemp to flat an array I employed the destructuring_assignment introduced in ES6.

// typeScriptArray:Array