Flatten an array of arrays in TypeScript

flatten.ts

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, []);
};