post_slide_img_to_discord.gs

const PRESENTATION_ID = "xxxpresentationidxxx"
const FOLDER_ID = "xxxfolderidxxx"
const DISOCRD_WEBHOOK_TOKEN = "xxxwebhooktokenxxx"
const DISCORD_WEBHOOK_URL = `https://discord.com/api/webhooks/xxxx/${DISOCRD_WEBHOOK_TOKEN}`

const main = () => {
  const slide = SlidesApp.openById(PRESENTATION_ID).getSlides()[0]
  const imgBlob = getImageBlob(slide, PRESENTATION_ID)
  const file = DriveApp.getFolderById(FOLDER_ID).createFile(imgBlob).setName("date.png")
  const imageUrl = updateAccessAndGetImageUrl(file)
  postToDiscord([{title: "スライドから画像が投稿できます！", image: {url: imageUrl}}])
}

/**
 * スライドから画像のBlob取得
 */
const getImageBlob = (silde, presentationId) => {
  const url = `https://docs.google.com/presentation/d/${presentationId}/export/png?id=${presentationId}&pageid=${silde.getObjectId()}`
  const options = {
    method: "get",
    headers: {"Authorization": `Bearer ${ScriptApp.getOAuthToken()}`},
    muteHttpExceptions: true
  }
  const response = UrlFetchApp.fetch(url, options)
  if (response.getResponseCode() === 200) {
    return response.getAs("image/png")
  } else {
    return null
  }
}

/**
 * 画像ファイルのアクセス権を更新して、閲覧できるURLを返す
 */
const updateAccessAndGetImageUrl = file => {
  file.setSharing(DriveApp.Access.ANYONE_WITH_LINK, DriveApp.Permission.VIEW)
  return `http://drive.google.com/uc?export=view&id=${file.getId()}`
}

/**
 * Webhook を使って Discord へ投稿する
 */
const postToDiscord = embeds => {
  const jsonData = {
    token: DISOCRD_WEBHOOK_TOKEN,
    embeds: embeds
  }
  const options = {
    method: "post",
    headers: {"Content-type": "application/json"},
    payload: JSON.stringify(jsonData)
  }
  UrlFetchApp.fetch(DISCORD_WEBHOOK_URL, options)
}