Check signup emails in Express

This guide adds a Signitize check to an Express signup route as a small middleware. It needs Node.js 18 or later (built-in fetch).

1. Add your key

.env

SIGNITIZE_API_KEY=sk_live_...

Get a free key if you don't have one.

2. The middleware

middleware/check-email.js

async function callSignitize(email) {
    try {
        const response = await fetch('https://api.signitize.io/v1/check', {
            method: 'POST',
            headers: {
                Authorization: `Bearer ${process.env.SIGNITIZE_API_KEY}`,
                'Content-Type': 'application/json',
            },
            body: JSON.stringify({ email }),
            signal: AbortSignal.timeout(5000),
        })
        if (response.ok) return { status: 'checked', ...(await response.json()) }
        if (response.status === 400) return { status: 'invalid' }
        if ([401, 422, 429].includes(response.status)) console.warn('Signitize answered', response.status)
    } catch {
        // timeout or network error
    }
    return { status: 'unavailable' }
}

export async function checkEmail(req, res, next) {
    const check = await callSignitize(String(req.body.email ?? '').trim())

    if (check.status === 'invalid') return res.status(422).json({ error: 'Enter a valid email address.' })
    if (check.status === 'checked' && check.disposable) {
        return res.status(422).json({ error: 'Please use a permanent email address.' })
    }
    if (check.status === 'checked' && !check.deliverable) {
        return res.status(422).json({ error: 'This email address cannot receive mail. Please check it for typos.' })
    }

    req.emailCheck = check
    next()
}

3. The route

routes/signup.js

import express from 'express'

import { checkEmail } from '../middleware/check-email.js'
import { users } from '../db.js'

const router = express.Router()

router.post('/signup', express.json(), checkEmail, async (req, res) => {
    const typed = req.body.email.trim()
    const check = req.emailCheck

    if (check.status === 'checked') {
        if (await users.findByCanonicalEmail(check.email)) {
            return res.status(409).json({ error: 'You already have an account.' })
        }
        await users.create({ email: typed, emailCanonical: check.email, emailCheck: 'passed' })
    } else {
        await users.create({ email: typed, emailCanonical: null, emailCheck: 'pending' })
    }

    await sendVerificationEmail(typed)
    res.status(201).json({ ok: true })
})

export default router

Store emailCanonical in a unique, nullable column, and send mail to the typed address.

With the npm client

The signitize package replaces callSignitize above. Its safeCheck() never throws and returns checked, invalid or unavailable:

npm install signitize

middleware/check-email.js

import { Signitize } from 'signitize'

const signitize = new Signitize() // reads SIGNITIZE_API_KEY

export async function checkEmail(req, res, next) {
    const outcome = await signitize.safeCheck(String(req.body.email ?? '').trim())

    if (outcome.status === 'invalid') return res.status(422).json({ error: 'Enter a valid email address.' })
    if (outcome.status === 'checked' && outcome.result.disposable) {
        return res.status(422).json({ error: 'Please use a permanent email address.' })
    }
    if (outcome.status === 'checked' && !outcome.result.deliverable) {
        return res.status(422).json({ error: 'This email address cannot receive mail. Please check it for typos.' })
    }

    req.emailCheck = outcome.status === 'checked' ? { status: 'checked', ...outcome.result } : { status: 'unavailable' }
    next()
}

The route in step 3 stays the same. A runnable version is in the Express example.

4. Recheck pending users

Users created while Signitize was unavailable are pending. Run the recheck job from Never block a signup every few minutes and grant free credits only to passed users.