Module 03 Tutorial · Feature Flag Service Track

What You'll Have at the End

Definition of Done

POST /flags/:id/rules — create a targeting rule with priority.
PATCH /flags/:id/rules/:ruleId — update rule condition or priority.
DELETE /flags/:id/rules/:ruleId — delete a rule.
Rule operators: eq, ne, contains, regex, gt, lt.
Rule types: segment membership, user attributes, percentage-based.
Rule priority: lower number = evaluated first.
Evaluation respects rule priority and returns matching rule ID.
Test suite: rule evaluation with multiple rules and priorities.

The Steps

Build It

STEP 1

Create rule evaluation engine

Create src/utils/ruleEngine.ts:

import { EvaluationContext } from '../types/flag';

interface Rule {
  rule_type: 'segment' | 'user_attribute' | 'percentage';
  rule_value: Record;
}

export function evaluateRule(rule: Rule, context: EvaluationContext): boolean {
  if (rule.rule_type === 'segment') {
    return context.segment === rule.rule_value.segment;
  }

  if (rule.rule_type === 'user_attribute') {
    const attr = rule.rule_value.attribute;
    const operator = rule.rule_value.operator; // eq, ne, contains, regex, gt, lt
    const expected = rule.rule_value.value;
    const actual = context.attributes?.[attr];

    if (actual === undefined) return false;

    switch (operator) {
      case 'eq':
        return actual === expected;
      case 'ne':
        return actual !== expected;
      case 'contains':
        return String(actual).includes(String(expected));
      case 'regex':
        return new RegExp(expected).test(String(actual));
      case 'gt':
        return Number(actual) > Number(expected);
      case 'lt':
        return Number(actual) < Number(expected);
      default:
        return false;
    }
  }

  if (rule.rule_type === 'percentage') {
    const percentage = rule.rule_value.percentage || 0;
    const userId = context.user_id || context.session_id || '';
    const hash = hashString(userId);
    return hash % 100 < percentage;
  }

  return false;
}

function hashString(str: string): number {
  let hash = 0;
  for (let i = 0; i < str.length; i++) {
    const char = str.charCodeAt(i);
    hash = (hash << 5) - hash + char;
    hash = hash & hash; // Convert to 32-bit integer
  }
  return Math.abs(hash);
}