.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

This can also work.

let arr = [ [1,2,3], [2,3,4] ];
console.log([].concat(...arr))

Or for older browsers,

[].concat.apply([], arr);