What does the main field in package.json do?

When you write require("fest-utils"), Node finds the folder node_modules/fest-utils and then asks, "Which file is the front door?" The main field is the answer. It is like the signboard at a college gate that tells visitors which building to enter first.

{
  "name": "fest-utils",
  "version": "2.0.0",
  "main": "lib/index.js"
}

With this, require("fest-utils") loads lib/index.js. If main is absent, Node tries index.js at the package root.

Two related points:

• For an application you run yourself, main matters little. You usually start it with node src/server.js or an npm script.
• The newer exports field gives more control: it can point to different files for require and import, and it blocks deep imports like fest-utils/lib/secret.js that you did not intend to publish. When exports exists, it takes priority over main.

{
  "exports": {
    ".": { "import": "./esm/index.js", "require": "./cjs/index.cjs" }
  }
}

• Pointing main at a file that is not included in the published package.
• Forgetting that exports overrides main.