util and structured clone in JavaScript

promisify for callback APIs, and a deep copy that handles cycles.

import { inspect, promisify } from "node:util";

function legacy(value, callback) {
  if (value < 0) return callback(new Error("negative"));
  return callback(null, value * 2);
}

const modern = promisify(legacy);
console.log(await modern(21));
console.log(await modern(-1).catch((error) => error.message));

const original = { when: new Date(0), tags: new Set(["a"]) };
original.self = original;
const copy = structuredClone(original);
console.log(copy.self === copy, copy.tags.has("a"), copy.when.getTime());

console.log(inspect({ nested: { deep: [1, 2] } }, { depth: 0 }));

How it works

1. promisify converts an error-first callback function.
2. structuredClone copies Maps, Sets, Dates and cycles.
3. util.inspect is what console.log uses underneath.

Keywords and builtins used here

• Date
• Set
• await
• catch
• const
• from
• function
• if
• import
• return

The run, in numbers

Lines

17

Characters to type

573

Tokens

179

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 102 seconds.

Type this snippet

Step 2 of 3 in The process, step 8 of 18 in Node.js.

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