firebase admin sdk send verification email

index.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');

admin.initializeApp({
    credential: admin.credential.applicationDefault()
});

// taken from here: https://stackoverflow.com/a/41886023/4139896

exports.testSendVerificationEmail = functions.https.onRequest(async (req, res) => {

    const firebase = require('firebase');

    var config = {
        /* PUT YOUR PROJECT'S WEB APP CONFIG HERE */
    };
    
    const app = firebase.initializeApp(config);

    var userData = {
        email: "[email protected]",
        emailVerified: false,
    };

    try {
        const user = await admin.auth().createUser(userData);
        const token = await admin.auth().createCustomToken(user.uid);
        const result = await firebase.auth().signInWithCustomToken(token);
        await result.user.sendEmailVerification();
        await firebase.auth().signOut();

        res.send("OK")

    } catch (err) {
        console.log("ERROR:", err);

        res.send("ERROR");
    }

});

Please give this a try. I can't test it right now but I believe I have incorporated all that was required. Oh and I copied the snippets from my code that is in Typescript not Javascript. So, u might have to make some corrections here and there.

import * as functions from 'firebase-functions';
import * as admin from 'firebase-admin';

// By default, firebase dependency is not included. I had to add it using instructions on the
// following link:
// https://stackoverflow.com/a/42846299/7983864
import { initializeApp } from 'firebase/app';
import { getAuth, signInWithCustomToken, sendEmailVerification } from 'firebase/auth';

// taken from here: https://stackoverflow.com/a/41886023/4139896

export const testSendVerificationEmail = functions.https.onRequest(async (req, res) => {

// const firebase = require('firebase');

var config = {
    /* PUT YOUR PROJECT'S WEB APP CONFIG HERE */
};

const app = initializeApp(config);

const auth = getAuth(app);

var userData = {
    email: "[email protected]",
    emailVerified: false,
};

try {
    const user = await admin.auth().createUser(userData);
    const token = await admin.auth().createCustomToken(user.uid);
    const result = await signInWithCustomToken(auth, token);
    await sendEmailVerification(result.user);
    await auth.signOut();

    res.send("OK")

} catch (err) {
    console.log("ERROR:", err);

    res.send("ERROR");
}

});

Hope it helps!