Very efficient type 4 UUID generator in vanilla JavaScript. Runs in ~7.81 cpb on an Intel Core i7-8650U @ 1.9 GHz.

index.js

const uuid = new UUID();
console.log(uuid.v4());

uuidv4.js

// Do not use for security. Use crypto.randomUUID() instead.
class UUID {
  #hex = [];

  constructor() {
    for (let i = 0; i < 0x10000; i++) {
      this.#hex[i] = i.toString(16).padStart(4, "0");
    }
  }

  v4() {
    const r1 = Math.random() * 0x100000000 >>> 0;
    const r2 = Math.random() * 0x100000000 >>> 0;
    const r3 = Math.random() * 0x100000000 >>> 0;
    const r4 = Math.random() * 0x100000000 >>> 0;

    return this.#hex[r1 >>> 16] + this.#hex[r1 & 0xffff] + "-" +// xxxxxxxx-....-4...-y...-............
      this.#hex[r2 >>> 16] + "-" +                              // ........-xxxx-4...-y...-............
      this.#hex[r2 & 0x0fff | 0x4000] + "-" +                   // ........-....-4xxx-y...-............
      this.#hex[r3 >>> 16 & 0x3fff | 0x8000] + "-" +            // ........-....-4...-yxxx-............
      this.#hex[r3 & 0xffff] +                                  // ........-....-4...-y...-xxxx........
      this.#hex[r4 >>> 16] + this.#hex[r4 & 0xffff];            // ........-....-4...-y...-....xxxxxxxx
  }
}