What if the code you are typing never ends?

Stop typing, and it runs. In real Node.

Your Snippet runs a moment after you stop typing, and the answer arrives in the console. Errors, values and timers are Node’s own.

Run as you type

A moment after you stop typing, the Snippet runs, and the answer appears in the console.

console.log(0.1 + 0.2)

1⌄console.log(0.1 + 0.2)

2⌄console.log(typeof null)

3⌄console.log("b" + "a" + +"a" + "a")

4⌄console.log(Math.max())

5⌄console.log(1)

6⌄setTimeout(() => console.log(2), 0)

7⌄Promise.resolve().then(() => console.log(3))

8⌄console.log(4)

0.30000000000000004
L1
object
L2
baNaNa
L3
-Infinity
L4
1
L5
4
L8
3
L7
2
L6

Done in 3ms·Runs as you type

Each line runs a moment after it is typed. The pasted block shows Node’s own order: 1, 4, 3, 2.

The last answer stays

While you type, the console keeps the last Run’s output, and the footer says the code changed since.

const user = { name: "Ada", langs: ["js", "ts"] }

1⌄const user = { name: "Ada", langs: ["js", "ts"] }

2⌄

3⌄console.log("Hi", 42)

4⌄console.log(user)

5⌄setTimeout(() => console.log("a moment later"), 100)

6⌄

7⌄user.langs.length

Hello 42
L3

  
  
Object
L4
<2
latea moment later
L5

Done in 103ms·code changed since·Runs as you type

The code now says Hi, but the console keeps the last Run’s Hello 42 until the next Run.

Node’s own errors

Errors and stack traces are V8’s, and each line of your code in them links to that line.

function total(items) {

1⌄function total(items) {

2⌄ return items.reduce((sum, item) => sum + item.price, 0)

3⌄}

4⌄

5⌄console.log(total([{ price: 5 }, { price: 7 }]))

6⌄console.log(total())

TypeError: Cannot read properties of undefined (reading 'reduce')

total — line 2

top level — line 6

Threw in 2ms·Runs as you type

reduce of undefined: each frame of the stack names its line, and a click goes there.

Values shown as they are

Map, Set, BigInt and circular objects print as they are.

const prices = new Map([["tea", 3], ["cake", 5]])

1⌄const prices = new Map([["tea", 3], ["cake", 5]])

2⌄const tags = new Set(["js", "ts", "js"])

3⌄const big = 2n ** 64n

4⌄const node = { name: "root" }

5⌄node.self = node

6⌄

7⌄console.log(prices)

8⌄console.log(tags)

9⌄console.log(big)

10⌄console.log(node)

  
  
Map(2)
L7
tea => 3
cake => 5

  
  
Set(2)
L8
0 : js
1 : ts
18446744073709551616n
L9

  
  
Object
L10
name : root
self : (already shown above)

Done in 5ms·Runs as you type

A Map, a Set, a BigInt and an object that contains itself, each printed as it is.

Every console method

console.table, groups, timers, warnings and %c styles work as in a browser, and each row ends with its line.

const langs = [

1⌄const langs = [

2⌄ { name: "JavaScript", year: 1995 },

3⌄ { name: "TypeScript", year: 2012 },

4⌄]

5⌄

6⌄console.table(langs)

7⌄

8⌄for (const lang of langs) console.log("checked")

9⌄

10⌄if (!langs.some((lang) => lang.name === "Rust")) {

11⌄ console.warn("No Rust yet")

12⌄}

13⌄console.error("Missing a year for", "Go")

  
Array(2)(index)nameyear
0JavaScript1995
1TypeScript2012
L6
2checked
L8

No Rust yet
L11

Missing a year for Go
L13

(index)	name	year
0	JavaScript	1995
1	TypeScript	2012

Done in 2ms·Runs as you type

A table, a line printed twice shown once with its count, a warning and an error.

Module or Script

A Snippet runs as an ES module or as CommonJS. When the code needs the other one, the console offers to switch.

const { basename, extname } = require("node:path")

1⌄const { basename, extname } = require("node:path")

2⌄

3⌄const photo = "/photos/2026/beach.jpeg"

4⌄

5⌄console.log(basename(photo, extname(photo)))

6⌄console.log(extname(photo))

ReferenceError: require is not defined in ES module scope, you can use import instead

top level — line 1

Switch to Script (CommonJS)

Threw in 2ms·Runs as you type

require in a Module throws. One click runs it again as a Script, and it prints beach and .jpeg.

A loop that never ends

The loop guard stops it and names the line. Click the line, fix it, and it runs. A Run busy for 5 seconds or using more than 1 GB is stopped too.

let tries = 0

1⌄let tries = 0

2⌄while (tries < 3) {

3⌄ // forgot tries++

4⌄ tries++

5⌄}

6⌄

7⌄console.log("done")

RangeError: Runaway loop on line 2: stopped after 20,000,000 iterations.

top level — line 2

Threw in 9ms·Runs as you type

tries++ was missing. The error names line 2, a click goes there, and the fix prints done.

Run longer JSpark+

Raise the time limit to 10 seconds and memory up to 8 GB, or give one Run more with Run longer.

let longest = 0

1⌄let longest = 0

2⌄let start = 0

3⌄

4⌄for (let n = 1; n < 5_000_000; n++) {

5⌄ let x = n

6⌄ let steps = 0

7⌄ while (x !== 1) {

8⌄ x = x % 2 ? 3 * x + 1 : x / 2

9⌄ steps++

10⌄ }

11⌄ if (steps > longest) {

12⌄ longest = steps

13⌄ start = n

14⌄ }

15⌄}

16⌄

17⌄console.log(start, "takes", longest, "steps")

Stopped: busy for 5.037s without a pause

It never paused for that long, which usually means an endless loop. Here’s what it got through.

Run longer

Stopped: busy for 5.0s·Runs as you type

Run longer finishes a search the 5-second limit stopped. Settings then raises the limit to 10 seconds.

Good to know

Your own access: A Node Run is real Node, with your access to your files, network and programs. Code that reaches them waits for you to press Run: see Node and your computer.
Node is inside the app: There is nothing to install. Settings → About names the Node, Chromium and V8 versions.
Stop at any time: Once a Run has lasted half a second, Stop takes the place of Run. ⌘. stops it too, and the console keeps what was printed.
Every top-level value: Turn on Settings → Console → Show every top-level value, and each top-level expression prints its value at its line.

Read the Run docs Read the Console docs Node and your computer All features

Stop typing, and the answer is there.

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