Modules & Local Use

JavaScript intermediate taught ES modules. In this download, map-limit.js exports mapLimit and TaskError; main.js imports the public function and demonstrates its output. package.json has type:module so Node treats .js files as ES modules. No installation is required because there are no dependencies.

Extract task-pool.zip, open a terminal inside task-pool and run npm start, then npm test. These are shell commands, not statements to paste into a console editor. Start prints 30,10,20. Tests report six grouped cases; each group contains several assertions and input cases.

The real module entry uses top-level await, which Node supports for ES modules. The existing JavaScript lesson editor instead uses async main() followed by main() because its runner initially parses a classic script. Preserve that wrapper for in-page async examples. The algorithm is the same; the entry point differs.

A reusable package also needs an environment policy. This teaching project targets modern browsers and Node 22+ and relies on AbortController, Error cause and performance.now. Publishing and compatibility with older engines are separate work; do not imply the download is a production package release.

Predict, run, and explain

Write the output and trace the important state changes before checking it. Then run the program. Each editor is independent. Async examples end with main() so this editor waits for their work.

const files = [
  "map-limit.js",
  "main.js",
  "pool.test.js",
  "package.json",
  "README.md"
];

console.log(files.length);
console.log(files.includes("map-limit.js"));

5
true

Follow source, entry point and tests separately

Open map-limit.js for the library contract, main.js for one caller and pool.test.js for controlled schedules. The demo chooses input and prints results; the library owns concurrency and cleanup. A successful demo is not evidence for every schedule. Keep its entry convention separate from the browser editor: Node ES modules can await at top level, while the classic-script editor uses async main followed by main().

Change only a demo input and predict its new result. Then identify the exported-function test catching a completion-order implementation. Keep generated output and dependencies outside authored source and rebuild the public archive after source edits.

Your turn

// Use accurate words, not a claim that cancellation can force arbitrary work to stop.

const api = {
  name: "mapLimit",
  resultOrder: "input",
  cancellation: "cooperative"
};

console.log(api.name);
console.log(api.resultOrder);
console.log(api.cancellation);

mapLimit
input
cooperative

Change it and test it

Run the download locally if possible, then change only main.js to use limit 1. Keep tests intact. Explain how you would tell a caller about finite-input and retained-output limits before they try a huge or infinite source.