Website change monitoring with scheduled screenshots: one page, one month

// watch.mjs: tell me when one element of one page changes.
// Node.js 18 or newer. No packages to install.
import { createHash } from 'node:crypto'
import { mkdir, readFile, rename, writeFile } from 'node:fs/promises'
import { pathToFileURL } from 'node:url'

const API = 'https://curlshot.com/api/v1'
const DIR = 'captures'
const OPTIONS = {
  url: 'https://example.com/pricing',
  selector: '#plans',
  wait_for_selector: '#plans',
  format: 'png',
  viewport_width: '1280',
  viewport_height: '1024',
  device_scale_factor: '1',
  block_cookie_banners: 'true',
  block_ads: 'true',
  block_chats: 'true',
  reduced_motion: 'true',
  time_zone: 'UTC',
}

export const sha256 = (bytes) => createHash('sha256').update(bytes).digest('hex')

// The whole decision, with no network and no disk in it.
// state.baseline is the hash of the last good capture,
// state.pending the hash of a change that was seen once.
export function decide(state, hash) {
  if (!state.baseline) return { event: 'first', state: { baseline: hash, pending: null } }
  if (hash === state.baseline) return { event: 'same', state: { baseline: state.baseline, pending: null } }
  if (hash !== state.pending) return { event: 'seen-once', state: { baseline: state.baseline, pending: hash } }
  return { event: 'changed', state: { baseline: hash, pending: null } }
}

async function capture() {
  const response = await fetch(API + '/screenshot?' + new URLSearchParams(OPTIONS), {
    headers: { 'X-Access-Key': process.env.SCREENSHOT_ACCESS_KEY ?? '' },
    signal: AbortSignal.timeout(90_000),
  })
  if (!response.ok) {
    throw new Error('Capture failed: HTTP ' + response.status + ' ' + (await response.text()).slice(0, 300))
  }
  return Buffer.from(await response.arrayBuffer())
}

async function notify(text) {
  console.log(text)
  const hook = process.env.ALERT_WEBHOOK_URL
  if (!hook) return
  await fetch(hook, {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ text }),
  })
}

async function main() {
  await mkdir(DIR, { recursive: true })
  const stateFile = DIR + '/state.json'
  const state = JSON.parse(await readFile(stateFile, 'utf8').catch(() => '{}'))

  const png = await capture()
  const result = decide(state, sha256(png))

  await writeFile(DIR + '/latest.png', png)
  if (result.event === 'changed') await rename(DIR + '/baseline.png', DIR + '/previous.png')
  if (result.event === 'first' || result.event === 'changed') await writeFile(DIR + '/baseline.png', png)
  await writeFile(stateFile, JSON.stringify(result.state, null, 2) + '\n')

  if (result.event === 'changed') {
    await notify(
      'Changed: ' + OPTIONS.url + ' ' + OPTIONS.selector +
        '. Compare ' + DIR + '/previous.png with ' + DIR + '/baseline.png.',
    )
  } else {
    const lines = {
      first: 'First capture saved as the baseline.',
      same: 'No change.',
      'seen-once': 'Different from the baseline. Waiting for the next run to confirm.',
    }
    console.log(lines[result.event])
  }
}

if (import.meta.url === pathToFileURL(process.argv[1] ?? '').href) {
  main().catch((error) => {
    console.error(error.message)
    process.exitCode = 1
  })
}