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Rules engine based on JSON Schema


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v1.2.0 - The one with events and named fact maps

  • Added testing to support named fact maps
  • Added testing for debug events



JSON Schema Rules Engine

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A highly configurable rules engine based on JSON Schema. Inspired by the popular JSON rules engine.

NBD: It actually doesn't have to use JSON Schema, but it's suggested


Three reasons:

  1. Schema validation of a data structure can be used to implement boolean logic
  2. Tools for JSON schema are everywhere and support is wide
  3. Custom operators (like those in JSON rules engine) aren't sustainable. You can either make a PR for a new operator that may or may not get merged OR you have to take on the ownership in your own codebase of building and maintaining custom operators. With json-schema-rules-engine, you can implement new logic immediately whenever the spec is published (thanks to very actively maintained projects like AJV).


  • Highly configurable - use any type of schema to express your logic (we strongly suggest JSON Schema)
  • Configurable interpolation to make highly reusable rules/actions
  • Zero-dependency, extremely lightweight (under 2kb minzipped)
  • Runs everywhere
  • Nested conditions allow for controlling rule evaluation order
  • Memoization makes it fast
  • No thrown errors - errors are emitted, never thrown


npm install json-schema-rules-engine # or yarn add json-schema-rules-engine

or, use it directly in the browser

<script src="https://cdn.jsdelivr.net/npm/json-schema-rules-engine"></script> <script> const engine = jsonSchemaRulesEngine(validator, { facts, actions, rules, }); </script>

Basic Example

import Ajv2019 from 'ajv/dist/2019'; import createRulesEngine from 'json-schema-rules-engine'; const facts = { weather: async ({ query, appId, units }) => { const url = `https://api.openweathermap.org/data/2.5/weather/?q=${q}&units=${units}&appid=${appId}`; return (await fetch(url)).json(); }, }; const rules = { dailyTemp: { when: [ { weather: { params: { query: '{{city}}', appId: '{{apiKey}}', units: '{{units}}', }, path: 'main.temp', is: { type: 'number', minimum: '{{hotTemp}}', }, }, }, ], then: { actions: [ { type: 'log', params: { message: 'Quite hot out today!' }, }, ], }, otherwise: { actions: [ { type: 'log', params: { message: 'Brrr, bundle up!' }, }, ], }, }, }; const actions = { log: console.log, }; // validate using a JSON schema via AJV const ajv = new Ajv(); const validator = async (subject, schema) => { const validate = await ajv.compile(schema); const result = validate(subject); return { result }; }; const engine = createRulesEngine(validator, { facts, rules, actions }); engine.run({ hotTemp: 20, city: 'Halifax', apiKey: 'XXXX', units: 'metric', }); // check the console



The validator is what makes json-schema-rules-engine so powerful. The validator is passed the resolved fact value and the schema (the value of the is property of an evaluator and asynchronously a ValidatorResult:

type ValidatorResult = { result: boolean; };

If you want to use json-schema-rules-engine as was originally envisioned - to allow encoding of boolean logic by means of JSON Schema - then this is a great validator to use:

import Ajv from 'Ajv'; const ajv = new Ajv(); const validator = async (subject, schema) => { const validate = await ajv.compile(schema); const result = validate(subject); return { result }; }; const engine = createRulesEngine(validator);

You can see by abstracting the JSON Schema part away from the core rules engine (by means of the validator) this engine can actually use anything to evaluate a property against. The validator is why json-schema-rules-engine is so small and so powerful.


context is the name of the object the rules engine evaluates during run. It can be used for interpolation or even as a source of facts

const context = { hotTemp: 20, city: 'Halifax', apiKey: 'XXXX', units: 'metric', }; engine.run(context);


There are two types of facts - static and functional. Functional facts come from the facts given to the rule engine when it is created (or via setFacts). They are unary functions that return a value, synchronously or asynchronously. Check out this example weather fact that calls an the openweather api and returns the JSON response.

const weather = async ({ query, appId, units }) => { const url = `https://api.openweathermap.org/data/2.5/weather/?q=${q}&units=${units}&appid=${appId}`; return (await fetch(url)).json(); };

Static facts are simply the values of the context object


It's important to note that all functional facts are memoized during an individual run of the rule engine - but not between runs - based on shallow equality of their argument. This is to ensure that the same functional fact can be evaluated in multiple rules without that fact being called more than once (useful for aysnchronous facts to prevent multiple API calls).

This means that functions that accept an argument that contains values that are objects or arrays are not memoized by default. But this can be configured using something like lodash's isEqual

import _ from 'lodash'; const engine = createRulesEngine(validator, { memoizer: _.isEqual });

If you want any of your facts to be memoized between runs, feel free to use our memoization helpers before setting the facts

import _ from 'lodash'; import { memo, memoRecord } from 'json-schema-rules-engine/memo'; // memoize a single function const memoizedFunction = memo((...args) => { /* ... */ }); // deep equal memoize const deeplyMemoizedFunction = memo((...args) => { /* ... */ }, _.isEqual); // memoize an object whos values are functions const memoizedFacts = memoRecord({ weather: async (...args) => { /* ... */ }, }); const deeplyMemoizedFacts = memoRecord( { weather: async (...args) => { /* ... */ }, }, _.isEqual, ); engine.setFacts(memoizedFacts);

If, for some reason, you do not want facts to be memoized during a run, then you can just pass a stub memoizer:

const engine = createRulesEngine(validator, { memoizer: () => false });


Actions, just like facts, are unary functions. They can be sync or async and can do anything. They are executed as an outcome of a rule.

const saveAuditRecord = async ({ eventType, data }) => { await db.insert('INSERT INTO audit_log (event, data) VALUES(?,?)', [ eventType, data, ]); }; const engine = createRulesEngine({ actions: saveAuditRecord });


Rules are written as when, then, otherwise. A when clause consists of an array of FactMaps, or an object whose values are FactMaps. If any of the FactMaps in the object or array evaluate to true, the properties of the then clause of the rule are evaluated. If not, the otherwise clause is evaluated.

const myRule = { when: [ { age: { is: { type: 'number', minimum: 30, }, }, name: { is: { type: 'string', pattern: '^J', }, }, }, ], then: { actions: [ { type: 'log', params: { message: 'Hi {{name}}!', }, }, ], }, }; const engine = createRulesEngine({ rules: { myRule } }); engine.run({ age: 31, name: 'Fred' }); // no action is fired engine.run({ age: 32, name: 'Joe' }); // fires the log action with { message: 'Hi Joe!' }

Nesting Rules

The then or otherwise property can consist of either actions, but it can also contain a nested rule. All functional facts in all FactMaps are evaluated simultaneously. By nesting when's, you can cause facts to be executed serially.

const myRule = { when: [ { weather: { params: { query: '{{city}}', appId: '{{apiKey}}', units: '{{units}}', }, path: 'main.temp', is: { type: 'number', minimum: 30 } }, }, ], then: { when: [ { forecast: { params: { appId: '{{apiKey}}', coord: '{{results[0].weather.value.coord}}' // interpolate a value returned from the first fact }, path: 'daily', is: { type: 'array', contains: { type: 'object', properties: { temp: { type: 'object', properties: { max: { type: 'number', minimum: 20 } } } } }, minContains: 4 } } }, then: { actions: { type: 'log', params: { message: 'Nice week of weather coming up', } } } ], actions: [ { type: 'log', params: { message: 'Warm one today', }, }, ], }, };


A FactMap is a plain object whose keys are facts (static or functional) and values are Evaluator's.


An evaluator is an object that specifies a JSON Schema to evaluate a fact against. If the fact is a functional fact, the evaluator can specify params to pass to the fact as an argument. A path can also be specified to more easily evaluate a nested property contained within the fact.

The following weather fact evaluator passes parameters to the function and specifies a schema to check the value at main.temp against:

const myFactMap = { weather: { params: { query: '{{city}}', appId: '{{apiKey}}', units: '{{units}}', }, path: 'main.temp', is: { type: 'number', minimum: '{{hotTemp}}', }, }, };


By default, json-schema-rules-engine uses dot notation - like property-expr or lodash's get - to retrieve an innter value from an object or array via path. This can be changed by the resolver option. For example, if you wanted to use json pointer, you could do it like this:

import { get } from 'jsonpointer'; const engine = createRulesEngine(validator, { resolver: get }); engine.setRules({ myRule: { weather: { params: { query: '{{/city}}', appId: '{{/apiKey}}', units: '{{/units}}', }, path: '/main/temp', is: { type: 'number', minimum: '{{/hotTemp}}', }, }, }, });

NOTE: the resolver is also used to retrieve values for interpolation. If using jsonpointer notation, this means that interpolations must be prefixed with a /.


Interpolation is configurable by passing the pattern option. By default, it uses the handlebars-style pattern of {{variable}}.

Anything passed in via the context object given to engine.run is available to be interpolated anywhere in a rule.

In addition to context, actions have a special property called results that can be used for interpolation in then and otherwise clauses.

Results Context

The (top level) when clause of a rule can interpolate things from context. But the then and otherwise have a special property available to them called results that you can interpolate. This is where defining FactMap as arrays or objects also comes into play. Consider the following rule:

const rules = { dailyTemp: { when: [ { weather: { params: { query: '{{city}}', appId: '{{apiKey}}', units: '{{units}}', }, path: 'main.temp', is: { type: 'number', minimum: '{{hotTemp}}', }, }, }, ], then: { actions: [ { type: 'log', params: { message: 'Quite hot out today - going to be {{results[0].weather.resolved}}!', }, }, ], }, otherwise: { actions: [ { type: 'log', params: { message: 'Brrr, bundle up - only going to be {{resilts[0].weather.resolved}}', }, }, ], }, }, };

If we were to name the FactMap using an object instead of an array, we could use the key of the FactMap for the interpolation:

const rules = { dailyTemp: { when: { myWeatherCondition: { weather: { params: { query: '{{city}}', appId: '{{apiKey}}', units: '{{units}}', }, path: 'main.temp', is: { type: 'number', minimum: '{{hotTemp}}', }, }, }, }, then: { actions: [ { type: 'log', params: { message: 'Quite hot out today - going to be {{results.myWeatherCondition.weather.resolved}}!', }, }, ], }, }, };

Two things to note:

  1. The results variable is local to the rule that it's operating in. Different rules have different results.
  2. There are two properties on the fact name (weather in the above case):
    • value - the value returned from the function (or the value from context if using a static fact)
    • resolved - the value being evaluated. If there is no path, value and resolved are the same


The rules engine is also an event emitter. There are 4 types of events you can listen to


Emitted as soon as you call run on the engine

engine.on('start', ({ context, facts, rules, actions }) => { /* ... */ });


Emitted when all rules have been evaluated AND all actions have been executed

engine.on('complete', ({ context, results }) => { /* ... */ });


Useful to monitor the internal execution and evaluation of facts and actions

engine.on('debug', ({ type, ...rest }) => { /* ... */ });


Any errors thrown during fact execution/evaluation or action execution are emitted via error

engine.on('error', ({ type, ...rest }) => { /* ... */ });

The errors that can be emitted are:

  • FactExecutionError - errors thrown during the execution of functional facts
  • FactEvaluationError - errors thrown during the evaluation of facts/results from facts
  • ActionExecutionError - errors thrown during the execution of actions


  • createRulesEngine(validator: Validator, options?: Options): RulesEngine
type Options = { facts?: Record<string,Fact>; rules?: Record<string,Rule>; actions?: Record<string,Action>; pattern?: RegExp; // for interpolation memoizer?: <T>(a: T, b: T) => boolean; resolver?: (subject: Record<string,any>, path: string) => any }; interface RulesEngine { setRules(rulesPatch: Patch<Rules>): void; setFacts(factsPatch: Patch<Facts>): void; setActions(actionsPatch: Patch<Actions>): void; on('debug', subscriber: DebugSubscriber): Unsubscribe on('error', subscriber: ErrorSubscriber): Unsubscribe on('start', subscriber: StartSubscriber): Unsubscribe on('complete', subscriber: CompleteSubscriber): Unsubscribe run(context: Record<string, any>): Promise<EngineResults>; } type Unsubscribe = () => void; type PatchFunction<T> = (o: T) => T; type Patch<T> = PatchFunction<T> | Partial<T>;




Help wanted! I'd like to create really great advanced types around the content of the facts, actions, and context given to the engine. Reach out @akmjenkins or [email protected]




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