fix: prevent undefined from overriding defaults in defineBKTConfig

[duyhungtnn]

Changes

• This PR fixed the bug "Undefined Values Override Defaults in defineBKTConfig"
• Also add a custom lint rule to prevent this issue in the future

Bug Report: Undefined Values Override Defaults in defineBKTConfig

Issue Summary

The defineBKTConfig function has a critical bug where the spread operator ...config can override default values with undefined, causing properties that should have defaults to become undefined and potentially break the application.

It was related to an issue found in the React Native SDK: the SDK is not using InMemoryStorage when AsyncStorage not available

Root Cause

Location: src/BKTConfig.ts, line 88

const result: BKTConfig = {
  featureTag: '',
  eventsFlushInterval: MINIMUM_FLUSH_INTERVAL_MILLIS,
  eventsMaxQueueSize: DEFAULT_MAX_QUEUE_SIZE,
  pollingInterval: DEFAULT_POLLING_INTERVAL_MILLIS,
  storageKeyPrefix: '',
  userAgent,
  fetch,
  storageFactory: createBKTStorage,
  ...config, // BUG: This can override defaults with undefined values
}

The spread operation happens after setting defaults, so any property in config that is explicitly set to undefined will override the intended default value.

Affected Properties

Properties with NO validation (can remain undefined):

• eventsMaxQueueSize - No check, can be undefined
• storageKeyPrefix - No check, can be undefined
• storageFactory - No check, can be undefined

Properties with inadequate validation:

• pollingInterval - Uses < MINIMUM_POLLING_INTERVAL_MILLIS check, but undefined < number is false, so undefined passes through
• eventsFlushInterval - Same issue as above

Properties with working validation:

• featureTag - Has explicit === undefined check
• userAgent - Has !result.userAgent check
• fetch - Has !result.fetch check that throws error

How to Reproduce

import { defineBKTConfig } from './src/BKTConfig'

// Example 1: storageFactory becomes undefined
const config1 = {
  apiKey: 'test-key',
  apiEndpoint: 'https://api.example.com',
  appVersion: '1.0.0',
  storageFactory: undefined, // This overrides the default createBKTStorage
}

const result1 = defineBKTConfig(config1)
console.log(result1.storageFactory) // undefined (should be createBKTStorage)

// Example 2: eventsMaxQueueSize becomes undefined
const config2 = {
  apiKey: 'test-key',
  apiEndpoint: 'https://api.example.com',
  appVersion: '1.0.0',
  eventsMaxQueueSize: undefined, // This overrides the default 50
}

const result2 = defineBKTConfig(config2)
console.log(result2.eventsMaxQueueSize) // undefined (should be 50)

// Example 3: pollingInterval becomes undefined (bypasses validation)
const config3 = {
  apiKey: 'test-key',
  apiEndpoint: 'https://api.example.com',
  appVersion: '1.0.0',
  pollingInterval: undefined, // This overrides the default, and undefined < 60000 is false
}

const result3 = defineBKTConfig(config3)
console.log(result3.pollingInterval) // undefined (should be 600000)

// Example 4: Multiple undefined values causing issues
const config4 = {
  apiKey: 'test-key',
  apiEndpoint: 'https://api.example.com',
  appVersion: '1.0.0',
  eventsMaxQueueSize: undefined,
  storageKeyPrefix: undefined,
  pollingInterval: undefined,
  eventsFlushInterval: undefined,
}

const result4 = defineBKTConfig(config4)
// All these properties will be undefined instead of their defaults:
console.log(result4.eventsMaxQueueSize)  // undefined (should be 50)
console.log(result4.storageKeyPrefix)    // undefined (should be '')
console.log(result4.pollingInterval)     // undefined (should be 600000)
console.log(result4.eventsFlushInterval) // undefined (should be 30000)

Impact

• Runtime errors when undefined properties are used
• Inconsistent behavior between properties
• Type safety violation (TypeScript thinks properties are non-nullable, but they can be undefined)
• Potential crashes when calling methods on undefined objects (e.g., storageFactory)

[Copilot]

Pull Request Overview

This PR fixes a critical bug in the defineBKTConfig function where undefined values in the configuration object could override default values due to improper use of the spread operator. The fix replaces the problematic spread pattern with explicit nullish coalescing operators and adds a custom ESLint rule to prevent similar issues in the future.

Key Changes:

• Fixed the defineBKTConfig function to properly handle undefined values using nullish coalescing
• Added comprehensive test coverage for the undefined value scenarios
• Implemented a custom ESLint rule no-spread-after-defaults to prevent similar bugs
• Applied code style improvements throughout the codebase

Reviewed Changes

Copilot reviewed 16 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/BKTConfig.ts	Main fix: replaced spread operator with explicit property assignment using nullish coalescing
test/BKTConfig.spec.ts	Added comprehensive tests for undefined value handling scenarios
eslint-rules/no-spread-after-defaults.js	Custom ESLint rule to prevent spread-after-defaults pattern
eslint-rules/no-spread-after-defaults.test.js	Test suite for the custom ESLint rule
eslint-rules/no-spread-after-defaults.md	Documentation for the custom ESLint rule
eslint.config.cjs	ESLint configuration updates to include custom rule and new formatting rules
src/internal/event/EventCreators.ts	Added ESLint disable comments for legitimate spread usage
Multiple test/internal files	Code style improvements using nullish coalescing and formatting

Files not reviewed (1)

• pnpm-lock.yaml: Language not supported

[cre8ivejp]

Thank you!