npm init -y
npm install --save-exact --save-dev esbuild
npm i @aws-sdk/client-s3

{
  "name": "aws-s3-metadata-edit",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "start": "esbuild index.ts --bundle --outfile=out.js && node out.js"
  },

import {
    S3Client,
    ListObjectsV2Command,
    CopyObjectCommand
} from "@aws-sdk/client-s3"


// ここは各自変更してください
const BUCKET_NAME = ""
const ACCESS_KEY_ID = ""
const SECRET_ACCESS_KEY = ""
// ここまで

async function main() {

    // 認証
    const s3Client = new S3Client({
        region: 'ap-northeast-1',
        credentials: {
            accessKeyId: ACCESS_KEY_ID,
            secretAccessKey: SECRET_ACCESS_KEY
        }
    })

    // 取得
    const response = await s3Client.send(
        new ListObjectsV2Command({ Bucket: BUCKET_NAME })
    )

    // 成功してれば
    if (response.Contents) {
        for (const obj of response.Contents) {

            // 今回は拡張子を入れているので、それから Content-Type を探す
            let mimeType = 'application/octet-stream'
            switch (obj.Key?.split('.')[1]) {
                case "jpg":
                    mimeType = 'image/jpeg'
                    break
                case "png":
                    mimeType = 'image/png'
                    break
            }

            // 実行
            await s3Client.send(
                new CopyObjectCommand({
                    Bucket: BUCKET_NAME,
                    CopySource: `${BUCKET_NAME}/${obj.Key}`,
                    Key: obj.Key,
                    ContentType: mimeType, // これ！！
                    MetadataDirective: 'REPLACE'
                })
            )

            console.log(`完了 ${obj.Key}`)
        }
    }
}

main() // エントリーポイントを呼び出す