Under The Hood
Every topic is explained at four depths — an ELI5 metaphor, a practical junior walkthrough, a senior architectural view, and the real under-the-hood mechanics. No fluff, no videos, just the machinery.
285 topics · press ⌘ K to search.
Topic library
Grouped by framework and stack.
Frontend
browser
15 topics- Back/Forward Cache (bfcache): In-Memory Freezing & BFCache Ineligibility
- Double-Keyed HTTP Cache Partitioning & RFC 5861 stale-while-revalidate
- Font Rendering: FOUT vs FOIT, font-display, and Metric Overrides
- HTML Parser, CSSOM Blocking, and RenderTree Construction
- Interaction to Next Paint (INP): Input Delay, Processing, and Presentation
bun
15 topics- bun build --compile: Packaging JSC, Assets, and Code into a Single Binary
- bun install: Content-Addressable Global Cache & Linux clonefile/reflink
- Bun Macros: with { type: 'macro' } Build-Time Code Evaluation
- bun test: Snapshot Engine, Native Mock Hoisting, and Jest Compatibility
- bun:ffi: Direct dlopen C Memory Pointers Without Compilation Bindings
css
15 topicshtmx
15 topics- Handling 4xx/5xx HTTP Statuses with HX-Retarget and HX-Reswap
- HTML5 Form Validation & htmx:validation:failed Event Hooks
- HTMX Security: CSRF Token Headers and Untrusted HTML Sanitization
- hx-boost: Progressive Enhancement SPA Anchor and Form Interception
- hx-get & hx-post: Declarative AJAX & HX-Request Headers
javascript
15 topics- AbortController & AbortSignal: Cascading Fetch/Listener Cleanup
- async/await: Generator De-Sugaring & Microtask Queue Steps
- Closures: Lexical Scope Environments & Heap Context Retention
- ESM Static Linking, Cyclic Graph Evaluation, and import() Runtime Chunks
- Event Loop: Microtasks (Promises) vs Macrotasks (Timers, I/O)
nextjs
15 topicsreact
15 topics- <form action> & useFormStatus: Form Lifecycle and Pending State
- Automatic Batching: Microtask Grouping Across Asynchronous Boundaries
- Fiber Reconciler: Fiber Node Linked List, Work Loop, and Time Slicing
- React Context: Sub-Tree Re-Render Cost and Selector Memoization
- React Portals: DOM Detachment vs Virtual Tree Event Bubbling
solidjs
15 topics- <Dynamic component={...}>: Element Switching Without Full Re-Creation
- <ErrorBoundary>: Reactive Owner Scope Error Capture and Fallback Swaps
- <For> vs <Index>: Keyed by Item Reference vs Keyed by Array Index
- createEffect: Synchronous vs Microtask Scheduling & Tracking Scopes
- createMemo: Lazy Cached Computation & Dirty Version Propagation
svelte
15 topicsvue
15 topics- <KeepAlive>: Component Instance Caching, LRU Eviction, and onActivated Hooks
- <Suspense> Boundary and defineAsyncComponent() Loading Orchestration
- <Teleport>: DOM Relocation Outside Component Root & Dynamic Targets
- Block Tree Optimization: Flattening Dynamic Nodes via openBlock & dynamicChildren
- Custom Directives: Element Lifecycle Hooks (created, mounted, updated, unmounted)
Backend
fastapi
15 topics- async def (Event Loop) vs def (anyio Worker Thread Pool Execution)
- Automatic OpenAPI & Swagger/ReDoc JSON Schema Generation Mechanics
- BackgroundTasks: Post-Response Async Task Execution vs Celery Queues
- CORSMiddleware & TrustedHostMiddleware: Header Validations and Host Header Attacks
- Custom RequestValidationError & HTTP Status Code Mapping
go
15 topics- context.Context: Done() Channel, Parent-Child Tree Cancellation
- defer Evolution: Open-Coded vs Stack-Allocated vs Heap-Allocated
- Escape Analysis: Pointer Escapes, Interface Wrapping, Stack Growth
- Go Generics: GC Shape Stenciling & Dictionary Method Passing
- Go Netpoller: OS epoll/kqueue Integration Without Blocking Threads
java
15 topicsnestjs
15 topics- Circular Dependencies: forwardRef() and Module Dependency Cycles
- CQRS Pattern: CommandBus, QueryBus, EventBus, and Sagas
- createParamDecorator(): Custom Parameter Decorators & Metadata Extraction
- Dynamic Modules: forRoot(), forFeature(), and ConfigurableModuleBuilder
- Exception Filters: @Catch(), HttpException Normalization, and Error Boundaries
nodejs
15 topics- AsyncLocalStorage: Context Propagation Across Async/Await Hops
- child_process: spawn Streaming vs exec Buffer vs fork IPC
- cluster Module: Master-Worker Round-Robin IPC Socket Handoffs
- diagnostics_channel: In-Process Pub/Sub Performance Telemetry
- ESM Static Graph Evaluation vs CommonJS Synchronous require Cache
python
15 topics- Asyncio TaskGroup, ExceptionGroups, and Cooperative Cancel Scopes
- C Extensions & CFFI: Bypassing the GIL in C/Rust Native Libraries
- Context Managers: __enter__ / __exit__, @contextmanager, and ExitStack
- CPython Bytecode: .pyc Files, dis Disassembly, and the Eval Frame Loop
- CPython Memory Allocator: pymalloc Arenas, Pools, and Size Classes
rust
15 topics- Drop Checker, Drop Glue, ManuallyDrop<T>, and std::mem::forget
- FFI: extern "C", bindgen, and Zero-Overhead C Interop
- Interior Mutability: Cell<T>, RefCell<T>, Mutex<T>, RwLock<T>
- Macro System: macro_rules! AST Matching vs Procedural Derive Macros (syn/quote)
- Memory Layout: Struct Padding, Field Reordering, #[repr(C)], #[repr(packed)]
springboot
15 topics- @EventListener, @TransactionalEventListener, ApplicationEventPublisher
- @Transactional: Propagation, Isolation, and Rollback Rules
- Auto-Configuration: @ConditionalOnClass, @ConditionalOnMissingBean
- Feign Client Declarative HTTP & Resilience4j Circuit Breakers
- Hibernate First-Level (PersistenceContext) vs Second-Level Cache
Concept matrix
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Recent updates
- :has() Relational Selector: Invalidation Cost & Engine Heuristics css
- @EventListener, @TransactionalEventListener, ApplicationEventPublisher springboot
- @scope Rule vs Shadow DOM Encapsulation (:host, ::part) css
- @Transactional: Propagation, Isolation, and Rollback Rules springboot
- +page.ts (Universal) vs +page.server.ts (Server-Only) Load svelte