gmail-hook/apps/api/src/mime.js

function decodeBase64Url(data) {
  if (!data) return "";
  const pad = data.length % 4 === 0 ? "" : "=".repeat(4 - (data.length % 4));
  return Buffer.from(
    data.replace(/-/g, "+").replace(/_/g, "/") + pad,
    "base64",
  ).toString("utf8");
}

function stripHtml(html) {
  return html
    .replace(/<style[\s\S]*?<\/style>/gi, "")
    .replace(/<script[\s\S]*?<\/script>/gi, "")
    .replace(/<br\s*\/?>/gi, "\n")
    .replace(/<\/p>/gi, "\n\n")
    .replace(/<\/div>/gi, "\n")
    .replace(/<\/tr>/gi, "\n")
    .replace(/<[^>]+>/g, "")
    .replace(/&nbsp;/g, " ")
    .replace(/&amp;/g, "&")
    .replace(/&lt;/g, "<")
    .replace(/&gt;/g, ">")
    .replace(/&quot;/g, '"')
    .replace(/&#39;/g, "'")
    .replace(/&#x27;/gi, "'")
    .replace(/\n{3,}/g, "\n\n")
    .trim();
}

function walkParts(payload, acc = { text: "", html: "" }) {
  if (!payload) return acc;
  const mime = payload.mimeType || "";
  const data = payload.body?.data;
  if (data) {
    if (mime === "text/plain") acc.text += decodeBase64Url(data);
    else if (mime === "text/html") acc.html += decodeBase64Url(data);
  }
  for (const part of payload.parts || []) {
    walkParts(part, acc);
  }
  return acc;
}

export function extractBody(payload) {
  const { text, html } = walkParts(payload);
  if (text.trim()) return text.trim();
  if (html.trim()) return stripHtml(html);
  return "";
}

export function header(payload, name) {
  const match = payload?.headers?.find(
    (h) => h.name?.toLowerCase() === name.toLowerCase(),
  );
  return match?.value || "";
}

export function summarizeMessage(message) {
  const payload = message.payload || {};
  return {
    id: message.id,
    threadId: message.threadId,
    from: header(payload, "From"),
    to: header(payload, "To"),
    subject: header(payload, "Subject") || "(no subject)",
    date: header(payload, "Date"),
    snippet: message.snippet || "",
  };
}