Documentation/Languages/JavaScript

Email testing with JavaScript

Zero dependencies, built on the runtime's own fetch. Drop it into Jest, Vitest, Playwright, Cypress or a Node script.

Before you begin

A MailFlat account
Free to start, no card. Every plan can open inboxes from the API.
An account API key
Agents → API keys in the dashboard. It looks like mf_live_… and goes in the X-API-Key header. Tick Read messages and Create and delete inboxes (inbox:read + inbox:manage): a new key can only read, so opening an inbox fails with 403 until you add the second one. Keep it in an environment variable, never in the repo. See API keys and authentication.
Node 18+ (or any fetch-capable runtime)
That is the whole toolchain requirement.

Install

JavaScript

npm i @mailflat/sdk

Package: @mailflat/sdk

Your first inbox and one-time code

The whole loop: open an address, let your app mail it, read the code back, clean up.

JavaScript

import { MailFlat } from "@mailflat/sdk";
 
const mf = new MailFlat({ apiKey: process.env.MAILFLAT_API_KEY });
 
// 1. a real, deliverable address
//    prefix = the local part of the address. label is a display name only.
const inbox = await mf.create({ prefix: "signup" });
console.log(inbox.address);          // signup@a7f2c.mailflat.net
 
// 2. your app sends the code to that address
await myApp.register({ email: inbox.address });
 
// 3. read it back, no mocking anywhere
const otp = await inbox.waitForOtp({ timeout: 30000 });
console.log(otp);                    // "482913"
 
// 4. done with it
await inbox.delete();

Line by linewhat each step of this JavaScript example does

create(...): Returns immediately with a real, deliverable address. Nothing is queued or simulated.
retention_hours: Optional. Messages purge themselves after it, so a skipped teardown never leaks.
wait_for_otp: Polls for you and fails loudly on timeout, instead of returning nothing three lines before the real error.
delete(): Optional but tidy. Retention would clean up anyway.

The address is real

Mail actually travels: SMTP, DKIM, the lot. Nothing is stubbed, so a broken template or a misconfigured sender fails here exactly like it would in production.

Waiting for the code

We extract the one-time code server-side and hand it to you as a field, so you never write a regex against an email body.

JavaScript

// timeout and pollInterval are MILLISECONDS here (the Python client uses seconds)
const otp = await inbox.waitForOtp({ timeout: 30000, pollInterval: 1000 });
 
// need the whole message?
const msg = await inbox.waitForMessage({ timeout: 30000 });
console.log(msg.subject, msg.sender, msg.text);
 
// both reject on timeout, so a missing email fails the test loudly
import { OTPTimeoutError } from "@mailflat/sdk";
try {
  await inbox.waitForOtp({ timeout: 5000 });
} catch (err) {
  if (err instanceof OTPTimeoutError) console.log("no code arrived");
}

Always set a timeout

A poll loop without a deadline turns a missing email into a hung job. Fail loudly instead: the error message should name the address you were waiting on.

Reading every message

JavaScript

for (const msg of await inbox.messages()) {   // received mail, newest first
  console.log(msg.subject, msg.sender, msg.receivedAt);
  console.log(msg.text);                     // plain text body
  console.log(msg.links);                    // URLs found in the body
  console.log(msg.otp);                      // extracted code, or undefined
}
 
// mail YOU sent from this address is not in the list above. Ask for it:
const sent = await inbox.messages({ direction: "out" });
 
const latest = await inbox.latest();         // null until the first message lands
if (latest?.subject?.includes("Reset")) {
  await latest.delete();                     // drop one message, keep the inbox
}

Field	Type	What it is	JSON field (msg.raw)
subject	string	Subject line	subject
sender	string	From address	sender
text	string	Plain text body	body_text
html	string	HTML body	body_html
otp	string \| null	One-time code, extracted by us	otp_code
toAddress	string	The exact address it was sent to, tag included	to_address
tag	string \| null	Plus-addressing tag, if the sender used one	tag
receivedAt	ISO 8601 UTC (ends with Z)	When it landed	received_at
isEncrypted	boolean	True on end-to-end encrypted inboxes, where body and code are unavailable	is_encrypted

Sending mail from the inbox

Useful in reverse: point your own inbound pipeline at a MailFlat address and check that it handles what arrives.

On the Free plan you can send to your own addresses (your account email or any of your inboxes). To send anywhere else, connect and verify a domain. Free includes one; Pro plans include more.

JavaScript

// mail leaves from the inbox address, DKIM-signed by our own MTA
const result = await inbox.send("someone@example.com", {
  subject: "Welcome",
  body: "Plain text body",
  html: "<p>Optional HTML body</p>",
  cc: ["team@example.com"],          // written into the headers
  bcc: ["archive@example.com"],      // in NO header, not even in its own copy
  attachments: [
    { filename: "invoice.pdf", content: pdfBytes },      // Uint8Array
    { filename: "note.txt", contentBase64: "aGVsbG8=" }, // or base64 you already have
  ],
});
 
// send() resolves when the mail is ACCEPTED (HTTP 202), not when it is delivered:
// delivery runs on a queue. The response carries the id you follow it with.
console.log(result.message_id, result.queued);
 
import { SendFailedError, SendTimeoutError } from "@mailflat/sdk";
try {
  const sent = await inbox.waitUntilSent(result.message_id, { timeout: 120_000 });
  console.log(sent.sendStatus);      // "sent" (or "unsigned" if DKIM was skipped)
} catch (err) {
  if (err instanceof SendFailedError) {
    console.log("permanently failed:", err.message);   // the queue gave up
  } else if (err instanceof SendTimeoutError) {
    // NOT a failure: the queue is still retrying. Sending again delivers it twice.
    console.log("still queued; or subscribe to the message.delivered webhook");
  }
}
 
// The SDK takes bytes, never a file path: it also runs in the browser, where there
// is no filesystem. In Node, read the file yourself:
//   attachments: [{ filename: "invoice.pdf", content: await readFile("invoice.pdf") }]
// Size and count depend on your plan. Read GET /api/plans instead of hard-coding.

Cleaning up

JavaScript

await inbox.delete();                  // inbox and every message, immediately
 
// or leave it: messages expire on their own at the retention you asked for
const inbox = await mf.create({ label: "ci", retentionHours: 2 });

Two safety nets, use both

Delete in teardown so the list stays readable, and set retention_hours so a crashed run still cleans itself up.

In a test suite

Works with Jest, Vitest, Playwright, Cypress and anything else that gives you a setup and teardown hook.

JavaScript

// tests/signup.spec.js: one isolated inbox per test, torn down even on failure
import { test, expect } from "@playwright/test";
import { MailFlat } from "@mailflat/sdk";
 
const mf = new MailFlat({ apiKey: process.env.MAILFLAT_API_KEY });
 
test("user can sign up with a real code", async ({ page }) => {
  const inbox = await mf.create({
    prefix: `ci-${Date.now().toString(36)}`,
    retentionHours: 2,
  });
 
  try {
    await page.goto("/signup");
    await page.fill("#email", inbox.address);
    await page.click("#submit");
 
    await page.fill("#code", await inbox.waitForOtp({ timeout: 60000 }));
    await expect(page).toHaveURL(/dashboard/);
  } finally {
    await inbox.delete();
  }
});

Line by linewhat each step of this JavaScript example does

unique prefix: One inbox per test, so parallel workers never read each other's mail.
teardown hook: Runs even when the test fails. That is exactly when inboxes get left behind.
real code, real email: No test-mode backdoor in your app: the path under test is the one your users take.

JavaScript reference

Method	What it does
mf.create({ prefix, label, subdomain, domain, retentionHours })	Open an inbox, returns Inbox
mf.list()	Every inbox this key opened
mf.inbox(address)	Attach to an existing address without an API call
inbox.waitForOtp({ timeout, pollInterval })	Poll until a code arrives, rejects with OTPTimeoutError
inbox.waitForMessage({ timeout })	Same, but resolves to the whole Message
inbox.messages() / inbox.latest()	All messages (newest first) / the newest one or null
inbox.send(to, { subject, body, html })	Send from this address, DKIM-signed
inbox.delete() / inbox.deleteMessage(id)	Drop the inbox / one message

Full endpoint reference, including error shapes and rate limits: Agent API and MCP.

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