Flatten an array in TypeScript

An array containing primitive values, objects and other arrays can be flattened using a recursive reduce function.

const flatten = < T = any > (arr: T[]) => {
  const reducer = < T = any > (prev: T[], curr: T | T[]) => {
    if (curr.constructor !== Array) {
      return [...prev, curr];
    }
    return curr.reduce(reducer, prev);
  };
  return arr.reduce(reducer, []);
};

For example:

const values = [1, 2, 3, 4, [5, 6], 7, [8, [9]]];
const flatValues = flatten(values);
console.log(flatValues);
>>> [1, 2, 3, 4, 5, 6, 7, 8, 9]

Github Gist