Nodemailer generateVerificationToken() not working

Environment

 System:
    OS: Linux 5.15 Ubuntu 24.04.1 LTS 24.04.1 LTS (Noble Numbat)
    CPU: (12) x64 Intel(R) Core(TM) i5-10600 CPU @ 3.30GHz
    Memory: 14.38 GB / 19.46 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 22.9.0 - ~/.nvm/versions/node/v22.9.0/bin/node
    npm: 10.8.3 - ~/.nvm/versions/node/v22.9.0/bin/npm
  npmPackages:
    @auth/mongodb-adapter: ^3.7.2 => 3.7.2 
    next: 14.2.13 => 14.2.13 
    next-auth: ^5.0.0-beta.25 => 5.0.0-beta.25 
    react: ^18 => 18.3.1

Reproduction URL

http://none

Describe the issue

While using the following codes

  Nodemailer({
        maxAge: 3 * 60, // 3 minutes
        server: {
          ....................
        },
        from: process.env.EMAIL_FROM,
        async generateVerificationToken () {
          return generateOTP.toString()
        },
      }),

function generateOTP() {
  return randomInt(100000, 999999);
};

The link received in email is

http://localhost:3000/api/auth/callback/nodemailer?callbackUrl=http%3A%2F%2Flocalhost%3A3000%2Fsignin&token=function+generateOTP%28%29+%7B%0A++++return+%280%2Ccrypto__WEBPACK_IMPORTED_MODULE_5__.randomInt%29%28100000%2C+999999%29%3B%0A%7D&email=me%40email.com

Notice it doesn't return the 6 digit code I generated

I followed the following instructions from docs

How to reproduce

As above

Expected behavior

Should show e.g. token=912343 in link sent via email, instead it's showing token=function()

We could not detect a valid reproduction link. Make sure to follow the bug report template carefully.

Why was this issue closed?

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The bug template that you filled out has a section called "Reproduction URL", which is where you should provide the link to the reproduction.

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What should I do?

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In general, assume that we should not go through a lengthy onboarding process at your company code only to be able to verify an issue.

My repository is private and cannot make it public

In most cases, a private repo will not be a sufficient minimal reproduction, as this codebase might contain a lot of unrelated parts that would make our investigation take longer. Please do not make it public. Instead, create a new repository using the templates above, adding the relevant code to reproduce the issue. Common things to look out for:

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If the issue is not related to a monorepo specifically, try to reproduce the issue without a complex monorepo setup

I did not open this issue, but it is relevant to me, what can I do to help?

Anyone experiencing the same issue is welcome to provide a minimal reproduction following the above steps by opening a new issue.

I think my reproduction is good enough, why aren't you looking into it quickly?

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nextauthjs / next-auth