Live Demo

How Does it Work?

You can read more about how a Predictor works under the hood here.

Important Info

Having trouble? See all known issues here
Use the predictor that matches the environment where the sequence was originally generated. Meaning, if it came from Chrome, use the Chrome predictor, etc....
We recommend at least 6 numbers in the initial sequence for Bun and Safari, and at least 4 numbers for Node, Chrome, Firefox, and Deno.
Breaking changes in v2.0.0! The V8 Predictor was deprecated! Use the predictor that matches your runtime instead.
In v3.0.0, native runtime support for Bun and Deno was added! You can run the Bun predictor natively in Bun, and the Deno predictor natively in Deno!

Installation

Node

npm i js-randomness-predictor
yarn add js-randomness-predictor
pnpm add js-randomness-predictor
# etc...

Bun

bun add js-randomness-predictor

Deno

deno add npm:js-randomness-predictor

Usage

ESM

import JSRandomnessPredictor from "js-randomness-predictor";

CJS

const JSRandomnessPredictor = require("js-randomness-predictor");

Deno

import JSRandomnessPredictor from "npm:js-randomness-predictor";

Frontend/Browser

Browser usage is a little painful. 😬 Please see here for more info THIS GUIDE INCUDES dev servers, eg. the dev servers that vite, webpack, etc.. offer.

Node Predictor

See known Node issues here

Since we're running in Node, you can dynamically generate the initial sequence by calling the node() method without any parameters. This will automatically produce a sequence behind the scenes. Alternatively, you can manually provide a sequence if you prefer.

Provide Your Own Sequence

const sequence = Array.from({ length: 4 }, Math.random);
const nodePredictor = JSRandomnessPredictor.node(sequence);
const nextPrediction = await nodePredictor.predictNext();
// We can programmatically verify since we are running in Node.
const isAccurate = nextPrediction === Math.random();

Automatically Generate Sequence

// Automatically creates sequence behind the scenes
const nodePredictor = JSRandomnessPredictor.node();
const nextPrediction = await nodePredictor.predictNext();
// We can programmatically verify since we are running in Node.
const isAccurate = nextPrediction === Math.random();

Targeting a Different Node.js Version

You can target Node.js versions that are either older or newer than your current version.

For example:

If you're currently running Node.js v24.x.x but want to predict values generated in v22.x.x
Or if you're on Node.js v18.x.x and want to predict values from a newer version like v20.x.x

You can do this via the setNodeVersion(version) method.
Essentially, setting the Node.js version tells the predictor: "The sequence I provided was generated using Node.js version X."

⚠️ The provided sequence (and expected sequence) must be generated in the matching Node.js version used in setNodeVersion(...)!

// Current Node.js: v24.x.x
const nodePredictor = JSRandomnessPredictor.node(sequenceFromNodeV22);
nodePredictor.setNodeVersion({ major: 22, minor: 0, patch: 0 });

const expectedPredictionsFromNodeV22 = [/* Copied from Node.js v22 */];
const nextPrediction = await nodePredictor.predictNext();
const isCorrect = expectedPredictionsFromNodeV22[0] === nextPrediction;

Bun Predictor

See known Bun issues here

🔥 As of v3.0.0, you can run the Bun predictor natively in Bun! 🔥

🚧 Only use "standalone" Math.random() calls in Bun (for now) - see here for more info 🚧

If you are running natively in Bun, you can either provide your own initial sequence, or allow us to create one behind the scenes for you. If you are using the Bun Predictor outside of Bun, you must provide a sequence that was generated in Bun and copied over!

Provide Your Own Sequence

❗ The initial sequence must contain at least 6 elements! ❗

// Sequence must contain at least 6 elements!
const sequence = [
  Math.random(), Math.random(), Math.random(),
  Math.random(), Math.random(), Math.random()
];
const bunPredictor = JSRandomnessPredictor.bun(sequence);
const nextPrediction = await bunPredictor.predictNext();
// We can programmatically verify since we are running in Bun.
const isAccurate = nextPrediction === Math.random();

Automatically Generate Sequence

IMPORTANT : must be running natively in Bun for this to work!

// Automatically creates sequence behind the scenes
const bunPredictor = JSRandomnessPredictor.bun();
const nextPrediction = await bunPredictor.predictNext();
// We can programmatically verify since we are running in Bun.
const isAccurate = nextPrediction === Math.random();

Deno Predictor

🔥 As of v3.0.0, you can run the Deno predictor natively in Deno! 🔥

If you are running natively in Deno, you can either provide your own initial sequence, or allow us to create one behind the scenes for you. If you are using the Deno Predictor outside of Deno, you must provide a sequence that was generated in Deno and copied over!

Provide Your Own Sequence

const sequence = Array.from({ length: 4 }, Math.random);
const denoPredictor = JSRandomnessPredictor.deno(sequence);
const nextPrediction = await denoPredictor.predictNext();
// We can programmatically verify since we are running in Deno.
const isAccurate = nextPrediction === Math.random();

Automatically Generate Sequence

IMPORTANT : must be running natively in Deno for this to work!

// Automatically creates sequence behind the scenes
const denoPredictor = JSRandomnessPredictor.deno();
const nextPrediction = await denoPredictor.predictNext();
// We can programmatically verify since we are running in Deno.
const isAccurate = nextPrediction === Math.random();

Chrome Predictor

See known Chrome issues here

const chromePredictor = JSRandomnessPredictor.chrome([...]);
const nextPrediction = await chromePredictor.predictNext();
// You'll need to manually verify accuracy.

Firefox Predictor

See known Firefox issues here

const firefoxPredictor = JSRandomnessPredictor.firefox([...]);
const nextPrediction = await firefoxPredictor.predictNext();
// You'll need to manually verify accuracy.

Safari Predictor

See known Safari issues here

❗ The initial sequence must contain at least 6 elements! ❗

🚧 Only use "standalone" Math.random() calls in Safari (for now) - see here for more info 🚧

// MUST HAVE AT LEAST 6 ELEMENTS IN SEQUENCE!
const sequence = [...];
const safariPredictor = JSRandomnessPredictor.safari(sequence);
const nextPrediction = await safariPredictor.predictNext();
// You'll need to manually verify accuracy.

Command Line Interface

Important info

Each number in the sequence should be separated by a space
Each flag has a shorthand equivalent

# To get full list of options
js-randomness-predictor --help

# You can use shorthand for flags.
js-randomness-predictor
  -e <environment>
  [-v <environment_version>]
  [-s <sequence...>]
  [-p <num_predictions>]
  [-x <export_path>]
  [-f <force_overwrite_export_file_or_export_path_creation>]

Global Usage

To make the CLI accessible system-wide, install this package globally using the appropriate global flag for your package manager.

npm i -g js-randomness-predictor

Non-Global Usage

You'll need to manually specify the path within a project that has this package installed.

# Pretend we are in a project that has this package installed.
$ node_modules/.bin/js-randomness-predictor [options]

Choosing a Runtime

By default, we execute the CLI in Node. If you would like, you can choose to execcute the CLI in Bun or Deno as well.

If you choose to run the CLI in Bun, you are not required to provide a --sequence if --environment bun as well
If you choose to run the CLI in Deno, you are not required to provide a --sequence if --environment deno as well
You'll need to set an env variable called JSRP_RUNTIME to be bun, deno, or node (is node by default)

# This will use the Bun runtime to run the CLI
$ JSRP_RUNTIME=bun js-randomness-predictor -e bun # `--sequence` not required
$ JSRP_RUNTIME=bun js-randomness-predictor -e deno # ERROR `--sequence` IS required

# This will use the Deno runtime to run the CLI
$ JSRP_RUNTIME=deno js-randomness-predictor -e deno # `--sequence` not required
$ JSRP_RUNTIME=deno js-randomness-predictor -e bun # ERROR `--sequence` IS required

# This will use the Node runtime to run the CLI
$ js-randomness-predictor [args]

Windows is a little different

# Via 'cmd'
C:\>set JSRP_RUNTIME=bun && js-randomness-predictor [...]

# Via 'PowerShell'
PS C:\> $env:JSRP_RUNTIME = "bun" ; js-randomness-predictor [...]

Export Predictor Results

If you want to export results to a file you can use the --export (or -x) switch. to provide an export path.

Export path is relative to the current working directory (the directory where you are currently running the CLI from)
The provided path must be to a .json file
If the provided file already exists we do not overwrite it, unless the --force switch is used
If the full path you provided does not exist, we do not create it, unless the --force switch is used

CLI Examples

Node

When no --sequence is provided, a sequence will be generated automatically based on the current runtime.

# Auto-generate sequence
$ js-randomness-predictor --environment node

# Provide your own sequence and prediction count
$ js-randomness-predictor --environment node --sequence 1 2 3 4
$ js-randomness-predictor --environment node --sequence 1 2 3 4 --predictions 15

Targeting a Different Node.js Version

You can target Node.js versions that are either older or newer than your current version.

For example:

If you're currently running Node.js v24.x.x but want to predict values generated in v22.x.x
Or if you're on Node.js v18.x.x and want to predict values from a newer version like v20.x.x

You can do this using the --env-version (or -v) flag.
Essentially, this flag tells the predictor: "The sequence I provided was generated using Node.js version X."

⚠️ Only the major version number is needed for --env-version value.

# Specify environment version explicitly
$ js-randomness-predictor --environment node --env-version 22 --sequence 1 2 3 4

# Shorthand version
$ js-randomness-predictor -e node -v 22 -s 1 2 3 4

⚠️ If you use --env-version with a version different from your current Node.js version, the --sequence flag is required:

# Current Node.js: v24.2.0
$ js-randomness-predictor -e node -v 22 # ERROR!