Block disposable email in Firebase Auth

The code

// functions/src/index.ts
import { beforeUserCreated, HttpsError } from "firebase-functions/v2/identity";
import { defineSecret } from "firebase-functions/params";

const piisendKey = defineSecret("PIISEND_API_KEY");
const PIISEND_URL = "https://api.piisend.com/api/v1/intelligence/email";

export const blockDisposable = beforeUserCreated(
  { secrets: [piisendKey] },
  async (event) => {
    const email = event.data.email;
    if (!email) return;

    try {
      const r = await fetch(PIISEND_URL, {
        method: "POST",
        headers: {
          Authorization: `Bearer ${piisendKey.value()}`,
          "Content-Type": "application/json",
        },
        body: JSON.stringify({ email }),
      });
      if (r.ok && (await r.json()).signals?.disposable) {
        throw new HttpsError("invalid-argument", "Please use a permanent email address.");
      }
    } catch (e) {
      if (e instanceof HttpsError) throw e;
      // fail open on network errors
    }
  },
);