function b(
  a                  // placeholder
){
  return a           // if the placeholder was passed
    ? (              // return a
      0 |            // floored
      Math.random()  // number
      * 16           // from 0 to 15
      ).toString(16) // in hexadecimal
    : (              // or otherwise a concatenated string:
      [1e7] +        // 10000000 +
      -1e3 +         // -1000 +
      -4e3 +         // -4000 +
      -8e3 +         // -80000000 +
      -1e11          // -100000000000,
      ).replace(     // replacing
        /1|0/g,      // ones and zeroes with
        b            // random hex digits
      )
}