README.md

npm pack # or npm publish
tar -Oxf blah-1.0.0.tgz package/package.json
# {
#   "name": "blah",
#   "version": "1.0.0",
#   "type": "module",
#   "exports": {
#     ".": {
#       "types": "./dist/index.d.ts",
#       "import": "./dist/index.mjs"
#     }
#   },
#   "scripts": {
#     "backup": "node backup.js",
#     "prepack": "npm run backup && node prepack.js",
#     "postpack": "node postpack.js"
#   }
# }

backup.js

import * as fs from "node:fs";

fs.writeFileSync("./package.json.backup", fs.readFileSync("./package.json"));

package.json

{
  "name": "blah",
  "version": "1.0.0",
  "type": "module",
  "exports": {
    ".": {
      "types": "index.d.ts",
      "import": "index.js"
    }
  },
  "scripts": {
    "backup": "node backup.js",
    "prepack": "npm run backup && node prepack.js",
    "postpack": "node postpack.js"
  }
}

postpack.js

import * as fs from "node:fs";

const backup = fs.readFileSync("./package.json.backup");
fs.writeFileSync("./package.json", backup);
fs.unlinkSync("./package.json.backup"); // delete backup file

prepack.js

import * as fs from "node:fs";

const contents = JSON.parse(fs.readFileSync("./package.json", { encoding: "utf8" }));
contents.exports["."] = {
	"types": "./dist/index.d.ts",
	"import": "./dist/index.mjs"
}
fs.writeFileSync("./package.json", JSON.stringify(contents, null, 2));