Rule Engine with Node.js

 

🚀 Building a Rule Engine with Node.js

By Sayyed Mohammad Adil

---

In today’s modern applications, handling complex conditions and business logic directly in the codebase can make systems rigid and hard to maintain. This is where a rule engine comes in handy — allowing us to separate rules from logic and make systems more flexible, scalable, and adaptable.

In this article, I’ll walk you through building a simple rule engine using Node.js, with support for conditional logic, custom actions, and contextual evaluation — all defined in JSON.

---

🧠 What Is a Rule Engine?

A rule engine is a system that evaluates a set of conditions (rules) against input data and determines which actions to execute. It’s widely used in:

• 🛒 E-commerce (discount rules, cart promotions)

• 🏦 Banking (loan eligibility, fraud detection)

• 🛂 Access control systems

• ⚙️ Workflow automation and approvals

By using a rule engine, you can let non-developers modify system behavior without touching application logic.

---

🏗️ Architecture Overview

Here's how a simple rule engine works:

1. Rule Definition – Rules written in JSON format.

2. Rule Parser – Interprets and validates each rule.

3. Executor – Evaluates conditions using actual input.

4. Result Handler – Returns decisions or triggers actions.

Let’s build it step by step 👇

---

🛠️ Step 1: Setup Your Node.js Project

bash
mkdir node-rule-engine
cd node-rule-engine
npm init -y
npm install lodash

---

📜 Step 2: Define Rules in JSON

json
// rules.json
[
  {
    "name": "High Order Discount",
    "condition": {
      "field": "orderAmount",
      "operator": ">",
      "value": 1000
    },
    "action": {
      "type": "DISCOUNT",
      "value": 10
    }
  },
  {
    "name": "First Time Buyer",
    "condition": {
      "field": "user.isNew",
      "operator": "==",
      "value": true
    },
    "action": {
      "type": "DISCOUNT",
      "value": 5
    }
  }
]

Each rule checks a condition and defines an action if true.

---

🧠 Step 3: Create the Rule Engine Logic

js
// ruleEngine.js
const _ = require('lodash');

const OPERATORS = {
  '==': (a, b) => a == b,
  '===': (a, b) => a === b,
  '!=': (a, b) => a != b,
  '>': (a, b) => a > b,
  '<': (a, b) => a < b,
  '>=': (a, b) => a >= b,
  '<=': (a, b) => a <= b
};

function evaluateCondition(condition, context) {
  const { field, operator, value } = condition;
  const data = _.get(context, field);
  const operation = OPERATORS[operator];
  return operation ? operation(data, value) : false;
}

function runRules(rules, context) {
  const actions = [];
  for (const rule of rules) {
    if (evaluateCondition(rule.condition, context)) {
      actions.push(rule.action);
    }
  }
  return actions;
}

module.exports = { runRules };

Here, we use lodash.get to safely access nested fields like user.isNew.

---

🧪 Step 4: Use the Engine in Your App

js
// app.js
const fs = require('fs');
const { runRules } = require('./ruleEngine');

const rules = JSON.parse(fs.readFileSync('./rules.json', 'utf8'));

const userContext = {
  orderAmount: 1200,
  user: {
    isNew: true
  }
};

const results = runRules(rules, userContext);
console.log('Triggered Actions:', results);

---

✅ Output:

bash
Triggered Actions: [
  { type: 'DISCOUNT', value: 10 },
  { type: 'DISCOUNT', value: 5 }
]

These can be used to apply dynamic pricing, inform your UI, or log decisions.

---

🧩 How to Extend This Rule Engine?

To make it more powerful, you can add:

• Logical grouping: AND, OR, NOT combinations.

• Rule prioritization (weight/priority field).

• Database-backed rules (MongoDB/PostgreSQL).

• Time-based rules (validity periods).

• UI for business users to manage rules.

---

🌍 Real-World Applications

Industry	Use Case
E-Commerce	Dynamic discount engine
Banking	Loan eligibility check
HR	Candidate filtering
Logistics	Route optimization
IoT/Devices	Rule-based automation triggers

---

🧠 Final Thoughts

Rule engines provide a clean and scalable way to handle business logic that changes frequently or varies per customer/context. By decoupling rules from code, your application becomes easier to update and maintain.

Using Node.js, we built a lightweight but extendable engine — ready for production or further enhancements.

---

🙌 Let’s Connect

If you enjoyed this or want help implementing rule engines in your own projects, let’s connect:

🔗 LinkedIn
💻 GitHub

---