index.ts
import * as line from "@line/bot-sdk";
import * as functions from "firebase-functions";

// 環境変数の設定
const config = {
  channelAccessToken: "",
  channelSecret: ""
};

const client = new line.Client(config);

export const lineWebhook = functions
  .region("asia-northeast1")
  .https.onRequest(async (req, res) => {
    // LINE BOTからのアクセスかどうか確認
    const signature = req.get("x-line-signature");
    if (
      !signature ||
      !line.validateSignature(req.rawBody, config.channelSecret, signature)
    ) {
      throw new line.SignatureValidationFailed(
        "signature validation failed",
        signature
      );
    }

    // eventをeventHander関数へ渡す
    Promise.all(req.body.events.map(eventHandler))
      .then((result) => res.json(result))
      .catch((err) => console.error(err));
  });

// この関数に処理を実装
const eventHandler = async (event: line.WebhookEvent) => {}

ユーザーがBOTに送信した画像などを受け取る関数を実装します。

index.ts
// message idを指定してbufferを取得
const getContentBuffer = (messageId: string): Promise<Buffer> => {
  return new Promise((resolve, reject) => {
    client.getMessageContent(messageId).then((stream) => {
      const content: Buffer[] = [];
      stream
        .on("data", (chunk) => {
          content.push(Buffer.from(chunk));
        })
        .on("error", reject)
        .on("end", () => {
          resolve(Buffer.concat(content));
        });
    });
  });
};

// 使用例
const eventHandler = async (event: line.WebhookEvent) => {
    // message id を渡す
    const buffer = await getContentBuffer(event.message.id);
}