Amazon S3 Presigned post

implementation.ts

// ON THE SERVER

import { generatePresignedPost } from "./s3";

const {url, fields} = await generatePresignedPost(...); // fill with your own parameters.

// send the above url, fields to the Client.

// ON THE CLIENT/BROWSER

// we must POST to the `url`, and set all the entries in `fields` as entries
// in our formdata body. The file to be uploaded must have the key `"file"`.

const fd = new FormData();
Object.keys(fields).map(k => fd.set(k, fields[k]));
fd.set("file", yourFile);

const res = await fetch(url, {
  headers: { "Content-Type": "multipart/form-data" },
  body: fd,
  method: "POST"
});

kebabCase.ts

// From https://youmightnotneed.com/lodash/

const re = /([0-9]+|([A-Z][a-z]+)|[a-z]+|([A-Z]+)(?![a-z]))/g;

const kebabCase = (str: string) =>
  (String(str ?? "").match(re) || []).map((x) => x.toLowerCase()).join("-");

export default kebabCase;

s3.ts

import {S3Client} from "@aws-sdk/client-s3";
import { createPresignedPost } from "@aws-sdk/s3-presigned-post";
import kebabCase from "./kebabCase";

const s3 = new S3Client({
  region: Bun.env.S3_REGION,
  endpoint: Bun.env.S3_ENDPOINT,
  forcePathStyle: false,
  credentials: {
    accessKeyId: Bun.env.S3_ACCESS_KEY!,
    secretAccessKey: Bun.env.S3_SECRET_ACCESS_KEY!,
  },
});


export function generatePresignedPost(
  key: string,
  type: string,
  metadata: Record<string, string>,
  maxSizeInBytes = 5 * 1024 * 1024, // 5MiB
  minSizeInBytes = 1
) {
  const formattedMetadata = Object.fromEntries(
    Object.entries(metadata).map((i) => [`x-amz-meta-${kebabCase(i[0])}`, i[1]])
  );
  const metadataConditions = Object.entries(formattedMetadata).map((i) => [
    "eq",
    ...i,
  ]) as ["eq", string, string][];
  return createPresignedPost(s3, {
    Bucket: Bun.env.S3_BUCKET!,
    Key: key,
    Conditions: [
      { acl: "public-read" },
      { bucket: Bun.env.S3_BUCKET! },
      ["starts-with", "$key", "some-prefix/"], // to make sure files are only uploaded to the some-prefix/ folder.
      { "Content-Type": type },
      ["content-length-range", minSizeInBytes, maxSizeInBytes],
      ...metadataConditions,
    ],
    Fields: { acl: "public-read", ...formattedMetadata },
    Expires: 600,
  });
}

export default s3;