function flattenReducer(acc, arr) {
  try {
    // This is faster but susceptible to `RangeError: Maximum call stack size exceeded`
    acc.push(...arr);
    return acc;
  } catch (err) {
    // Fallback to a slower but safer option
    return acc.concat(arr);
  }
}

let start = performance.now();
Array(100).fill(Array(1000).fill(1)).reduce(flattenReducer);
let end = performance.now();
console.log(`Time: ${end - start} ms.`);


start = performance.now();
Array(100).fill(Array(1000).fill(1)).flat();
end = performance.now();
console.log(`Time: ${end - start} ms.`);

function fastJoin(arr, separator) {
  let isFirst = true;
  return arr.reduce((acc, elem) => {
    if (elem === null || elem === undefined) {
      elem = '';
    }

    if (isFirst) {
      isFirst = false;
      return `${elem}`;
    }

    return `${acc}${separator}${elem}`;
  }, '');
}

let size = 1000
let start = performance.now();
fastJoin(Array(size).fill('test'), '-');
let end = performance.now();
console.log(`Time: ${end - start} ms.`);


start = performance.now();
Array(size).fill('test').join('-');
end = performance.now();
console.log(`Time: ${end - start} ms.`);