Javascript rules engine

Usage no npm install needed!

<script type="module">
  import regent from '';


Regent: Business Rules in JavaScript

regent logo (the letter R wearing a crown)

Regent lets you query a data structure by asking true or false questions. Regent logic is written as single responsibility rules that are self-documenting, composable, and human readable. Let's look at an example.

import { equals } from 'regent';

// return a function that evaluates to true
// if prop `isRaining` is equal to true
const isRaining = equals('@isRaining', true);

// Define your data structure
const data = { isRaining: true };

// Evaluate the rule
const isUmbrellaNeeded = isRaining(data); // true

equals predicate documentation

Taking the previous example a bit further, we can refine the scenario to be more precise. We can create and combine multiple rules to test this condition

If it is raining and the wind isn't so strong the umbrella will turn inside-out and blow out of our hands, we need an umbrella.

import { and, equals, lessThan } from 'regent';

// Define a rule for `isRaining` and a rule for `isCalm`.
// Use regent and to compose them together
const isRaining = equals('@isRaining', true);
const isCalm = lessThan('@windSpeedInMph', 25);
const isRainingAndCalm = and(isRaining, isCalm);

// Define your data structure
const data = { isRaining: true, windSpeedInMph: 20 };

// Evaluate the rule
isRainingAndCalm(data); // true

composition documentation

Regent also provides a simple way to find or filter array items based on a regent rule.

{ equals, lessThan, find, filter } from 'regent';

const isRaining = equals('@isRaining', true);
const isSunny = lessThan('@cloudCover', 10);
const isCold = lessThan('@temperature', 60);

const data = {
  isRaining: false,
  cloudCover: 0,
  temperature: 42

const items = [
  { item: 'Winter hat', rule: isCold },
  { item: 'Sunglasses', rule: isSunny },
  { item: 'Umbrella', rule: isRaining },

// filter will return an array with the Winter Hat and Sunglasses objects
// because it is cold and sunny
// [{ item: 'Winter hat', rule: isCold }, { item: 'Sunglasses', rule: isSunny }]
filter(items, data)


npm install --save regent