nodejs buffer string write performance

write.js

'use strict';


function writeUTF8ToBuf(str, buf, off=0) {
   for (let i=0; i < str.length; i++) {
       const charcode = str.charCodeAt(i);
       if (charcode < 0x80) buf[off++] = charcode;
       else if (charcode < 0x800) {
         buf[off++] = 0xc0 | (charcode >> 6); 
         buf[off++] = 0x80 | (charcode & 0x3f);
       }
       else if (charcode < 0xd800 || charcode >= 0xe000) {
         buf[off++] = 0xe0 | (charcode >> 12);
         buf[off++] = 0x80 | ((charcode>>6) & 0x3f);
         buf[off++] = 0x80 | (charcode & 0x3f);
       }
       // surrogate pair
       else {
           i++;
           // UTF-16 encodes 0x10000-0x10FFFF by
           // subtracting 0x10000 and splitting the
           // 20 bits of 0x0-0xFFFFF into two halves
           charcode = 0x10000 + (((charcode & 0x3ff)<<10) | (str.charCodeAt(i) & 0x3ff));
            buf[off++] = 0xf0 | (charcode >>18);
            buf[off++] = 0x80 | ((charcode>>12) & 0x3f);
            buf[off++] = 0x80 | ((charcode>>6) & 0x3f);
            buf[off++] = 0x80 | (charcode & 0x3f);
       }
   }
   return off;
}

const buf = Buffer.alloc(4096);
const count = 1e7;

console.log('StrLen,writeUTF8ToBuf,Buffer.utf8Write,Buffer.write')
for(let k=0; k<11; k++) {
   const str = 'a'.repeat(Math.pow(2, k));
   const times = [str.length];
   let start = Date.now();
   for (let i = 0; i < count; ++i){
      writeUTF8ToBuf(str, buf);
   }
   times.push(Date.now() - start);

   start = Date.now();
   for (let i = 0; i < count; ++i){
      buf.utf8Write(str);
   }
   times.push(Date.now() - start);

   start = Date.now();
   for (let i = 0; i < count; ++i){
      buf.write(str);
   }
   times.push(Date.now() - start);
   console.log(times.join(','))
}

Used your gist for mongodb/js-bson#490