getStringImpl join array of strings vs string concat

bench.js

let buffer;
const LENGTH = 1024 * 4 + 17;
const WARMUP_COUNT = 8;

function createStringBuffer(len = LENGTH) {
  buffer = new ArrayBuffer(len * 2);
  const U16 = new Uint16Array(buffer);
  const alphabet = 'A BCDE, FGHI JKLM.; (NOPQ- RSTU, VWXYZ. abcd; efgh,) ijkl mnop. qrst- uvwxy,( z0123 4567, 89.)';
  for (var i = 0; i < len; i++) {
    U16[i] = alphabet.charCodeAt(Math.floor(Math.random() * alphabet.length));
  }
}

const CHUNKSIZE = 1024;

function getStringImpl_orig(buffer, ptr = 0, length = LENGTH) {
  // const U32 = new Uint32Array(buffer);
  const U16 = new Uint16Array(buffer);
  let offset = ptr >>> 1;
  if (length <= CHUNKSIZE) return String.fromCharCode.apply(String, U16.subarray(offset, offset + length));
  const parts = [];
  do {
    const last = U16[offset + CHUNKSIZE - 1];
    const size = last >= 0xD800 && last < 0xDC00 ? CHUNKSIZE - 1 : CHUNKSIZE;
    parts.push(String.fromCharCode.apply(String, U16.subarray(offset, offset += size)));
    length -= size;
  } while (length > CHUNKSIZE);
  return parts.join("") + String.fromCharCode.apply(String, U16.subarray(offset, offset + length));
}

function getStringImpl_new(buffer, ptr = 0, length = LENGTH) {
  // const U32 = new Uint32Array(buffer);
  const U16 = new Uint16Array(buffer);
  let offset = ptr >>> 1;
  if (length <= CHUNKSIZE) return String.fromCharCode.apply(String, U16.subarray(offset, offset + length));
  let parts = '';
  do {
    const last = U16[offset + CHUNKSIZE - 1];
    const size = last >= 0xD800 && last < 0xDC00 ? CHUNKSIZE - 1 : CHUNKSIZE;
    parts += String.fromCharCode.apply(String, U16.subarray(offset, offset += size));
    length -= size;
  } while (length > CHUNKSIZE);
  return parts + String.fromCharCode.apply(String, U16.subarray(offset, offset + length));
}

createStringBuffer();

// warmup
for (let i = 0; i < WARMUP_COUNT; i++) {
  getStringImpl_orig(buffer);
}

console.time('getStringImpl (array join)');
getStringImpl_orig(buffer);
console.timeEnd('getStringImpl (array join)');


for (let i = 0; i < WARMUP_COUNT; i++) {
  getStringImpl_new(buffer);
}

console.time('getStringImpl (string concat)');
getStringImpl_new(buffer);
console.timeEnd('getStringImpl (string concat)');

Results for node.js 14.2.0:

getStringImpl (array join):    0.109ms
getStringImpl (string concat): 0.03ms