Helpers for json-rule-engine

example.ts

import { processRules, RuleEngineError } from "./ruleEngineHelpers.ts";

const rules = [ruleA, ruleB, ruleC];

const facts = {
  fact1: "Some data",
  fact2: "Some other data",
  fact3: 42
};

const { errors, events } = await processRules([rule1, rule2, rule3], facts);

if (errors.length > 0) {
  throw new RuleEngineError(errors);
}

ruleEngineHelpers.ts

import {
  Rule,
  Engine,
  RuleResult,
  AllConditions,
  Almanac,
  AnyConditions,
  Event,
  NestedCondition,
  ConditionProperties
} from 'json-rules-engine';

export async function processRules<T>(rules: Rule[], facts: T) {
  const errors = [];
  const engine = new Engine(rules).on(
    'failure',
    async (event, almanac, ruleResult) => {
      errors.push({
        type: event.type,
        failedConditions: await getFailedConditions(almanac, ruleResult)
      });
    }
  );

  const result = await engine.run(facts);
  return {
    ...result,
    errors
  };
}

async function getFailedConditions(almanac: Almanac, ruleResult: RuleResult) {
  const allConditions = (ruleResult.conditions as AllConditions).all || [];
  const anyConditions = (ruleResult.conditions as AnyConditions).any || [];

  const conditions = [
    ...allConditions,
    ...anyConditions
  ] as (ConditionProperties & { result: unknown; factResult: unknown })[];
  if (conditions.length === 0) {
    return [];
  }

  return Promise.all(
    conditions
      .filter(c => !c.result)
      .map(async c => {
        const fact = c.fact;
        const expected = c.value.fact
          ? await almanac.factValue(c.value.fact)
          : c.value;
        const actual = c.factResult;
        return {
          message: `condition for ${fact} failed: expected '${expected}', actual '${actual}'`,
          fact,
          expected,
          actual
        };
      })
  );
}

export class RuleEngineError extends Error {
  constructor(errors) {
    const message = errors
      .map(
        e => `${e.type}[${e.failedConditions.map(c => c.message).join(',')}]`
      )
      .join('; ');

    super(message);
  }
}