Javascript random characters

// Asynchronous
const crypto = require('crypto');
 
function getRandomChars(len, callback) {
   const CHARS = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
   const NUM_CHARS = CHARS.length;
   crypto.randomBytes(len, (err, buf) => {
      let result = "";
      buf.forEach(b => {
         let idx = Math.floor(b * NUM_CHARS / 256);
         result += CHARS[idx];
      });
      callback(null, result);
   });
}