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engine.md

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Engine

The Engine stores and executes rules, emits events, and maintains state.

Methods

constructor([Array rules], Object [options])

let Engine = require('json-rules-engine').Engine

let engine = new Engine()

// initialize with rules
let engine = new Engine([Array rules])

// initialize with options
let options = {
  allowUndefinedFacts: false,
  pathResolver: (object, path) => _.get(object, path)
};
let engine = new Engine([Array rules], options)

Options

allowUndefinedFacts - By default, when a running engine encounters an undefined fact, an exception is thrown. Turning this option on will cause the engine to treat undefined facts as undefined. (default: false)

pathResolver - Allows a custom object path resolution library to be used. (default: json-path syntax). See custom path resolver docs.

engine.addFact(String id, Function [definitionFunc], Object [options])

// constant facts:
engine.addFact('speed-of-light', 299792458)

// facts computed via function
engine.addFact('account-type', function getAccountType(params, almanac) {
  // ...
})

// facts with options:
engine.addFact('account-type', function getAccountType(params, almanac) {
  // ...
}, { cache: false, priority: 500 })

engine.removeFact(String id)

engine.addFact('speed-of-light', 299792458)

// removes the fact
engine.removeFact('speed-of-light')

engine.addRule(Rule instance|Object options)

Adds a rule to the engine. The engine will execute the rule upon the next run()

let Rule = require('json-rules-engine').Rule

// via rule properties:
engine.addRule({
  conditions: {},
  event: {},
  priority: 1,                             // optional, default: 1
  onSuccess: function (event, almanac) {}, // optional
  onFailure: function (event, almanac) {}, // optional
})

// or rule instance:
let rule = new Rule()
engine.addRule(rule)

engine.removeRule(Rule instance)

Removes a rule from the engine.

// adds a rule
let rule = new Rule()
engine.addRule(rule)

//remove it
engine.removeRule(rule)

engine.addOperator(String operatorName, Function evaluateFunc(factValue, jsonValue))

Adds a custom operator to the engine. For situations that require going beyond the generic, built-in operators (equal, greaterThan, etc).

/*
 * operatorName - operator identifier mentioned in the rule condition
 * evaluateFunc(factValue, jsonValue) - compares fact result to the condition 'value', returning boolean
 *    factValue - the value returned from the fact
 *    jsonValue - the "value" property stored in the condition itself
 */
engine.addOperator('startsWithLetter', (factValue, jsonValue) => {
  if (!factValue.length) return false
  return factValue[0].toLowerCase() === jsonValue.toLowerCase()
})

// and to use the operator...
let rule = new Rule(
  conditions: {
    all: [
      {
        fact: 'username',
        operator: 'startsWithLetter', // reference the operator name in the rule
        value: 'a'
      }
    ]
  }
)

See the operator example

engine.removeOperator(String operatorName)

Removes a operator from the engine

engine.addOperator('startsWithLetter', (factValue, jsonValue) => {
  if (!factValue.length) return false
  return factValue[0].toLowerCase() === jsonValue.toLowerCase()
})

engine.removeOperator('startsWithLetter');

engine.run([Object facts], [Object options]) -> Promise ({ events: [], failureEvents: [], almanac: Almanac, results: [], failureResults: []})

Runs the rules engine. Returns a promise which resolves when all rules have been run.

// run the engine
await engine.run()

// with constant facts
await engine.run({ userId: 1 })

const {
  results,         // rule results for successful rules
  failureResults,  // rule results for failed rules
  events,          // array of successful rule events
  failureEvents,   // array of failed rule events
  almanac          // Almanac instance representing the run
} = await engine.run({ userId: 1 })

Link to the Almanac documentation

engine.stop() -> Engine

Stops the rules engine from running the next priority set of Rules. All remaining rules will be resolved as undefined, and no further events emitted.

Be aware that since rules of the same priority are evaluated in parallel(not series), other rules of the same priority may still emit events, even though the engine has been told to stop.

engine.stop()

There are two generic event emissions that trigger automatically:

engine.on('success', Function(Object event, Almanac almanac, RuleResult ruleResult))

Fires when a rule passes. The callback will receive the event object, the current Almanac, and the Rule Result. Any promise returned by the callback will be waited on to resolve before execution continues.

engine.on('success', function(event, almanac, ruleResult) {
  console.log(event) // { type: 'my-event', params: { id: 1 }
})

engine.on('failure', Function(Object event, Almanac almanac, RuleResult ruleResult))

Companion to 'success', except fires when a rule fails. The callback will receive the event object, the current Almanac, and the Rule Result. Any promise returned by the callback will be waited on to resolve before execution continues.

engine.on('failure', function(event, almanac, ruleResult) {
  console.log(event) // { type: 'my-event', params: { id: 1 }
})