Playground

The real playground is one command

Typing node with no arguments gives you the > prompt, which evaluates each line as you type it. It is the fastest way to answer "what does this actually do". .exit or Ctrl+D leaves it.

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8$ node
Welcome to Node.js v26.7.0.
Type ".help" for more information.
> 0.1 + 0.2
0.30000000000000004
> [1, 2, 3].map((n) => n * 2)
[ 2, 4, 6 ]
> .exit

For anything longer than a line or two, put it in a file and run that instead: node scratch.mjs. node --watch scratch.mjs re-runs it every time you save, which is the closest thing to a live playground there is.

A scratchpad that runs here

Edit it and press Run. It has the same check() the lessons use.

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10// Try something. check(name, got, want) prints ok or FAIL.
const counts = (words) => {
  const tally = new Map();
  for (const w of words) tally.set(w, (tally.get(w) ?? 0) + 1);
  return tally;
};

const result = counts(["a", "b", "a"]);
console.log(result);
check("counting words", result, new Map([["a", 2], ["b", 1]]));

Map(2) { 'a' => 2, 'b' => 1 }
ok   counting words