New and substantially updated lessons and reference entries.https://js.stuartsm.com/enhttps://js.stuartsm.com/learn/beginner/orientation-and-execution/https://js.stuartsm.com/learn/beginner/orientation-and-execution/What JavaScript is, where it runs, and how to read the statements, expressions, and errors in a small program.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/values-variables-and-operators/https://js.stuartsm.com/learn/beginner/values-variables-and-operators/The primitive types JavaScript works with, how to declare and name bindings, and the operators and equality rules that combine values into new ones.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/control-flow/https://js.stuartsm.com/learn/beginner/control-flow/Directing which statements run, and how many times, with conditionals and loops — plus writing both so they stay readable as they grow.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/functions-and-scope/https://js.stuartsm.com/learn/beginner/functions-and-scope/Three ways to write a function, how parameters and return values work, what makes a function pure, and a first mental model for closures.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/arrays-and-objects/https://js.stuartsm.com/learn/beginner/arrays-and-objects/Creating, reading, updating, copying, and transforming the two structures nearly every JavaScript program is built from.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/strings-numbers-and-dates/https://js.stuartsm.com/learn/beginner/strings-numbers-and-dates/The everyday string and number methods, locale-aware formatting, working with dates safely, and what problems regular expressions solve.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/errors-and-debugging/https://js.stuartsm.com/learn/beginner/errors-and-debugging/The four kinds of error you'll actually hit, how to read a stack trace, throwing and catching your own errors, and a repeatable process for finding a bug.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/the-browser-and-the-dom/https://js.stuartsm.com/learn/beginner/the-browser-and-the-dom/Selecting and updating elements, handling events safely, working with forms, and persisting small amounts of data in localStorage.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/async-basics-and-fetching/https://js.stuartsm.com/learn/beginner/async-basics-and-fetching/Why waiting for something needs a different execution model, how promises and async/await represent an eventual outcome, and fetching data with resilient loading/empty/error states.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/beginner/modules-tooling-and-first-tests/https://js.stuartsm.com/learn/beginner/modules-tooling-and-first-tests/Splitting code across files with export/import, what package.json and a lockfile actually do, and writing a first unit test with Arrange/Act/Assert.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/execution-model-scope-and-closures/https://js.stuartsm.com/learn/intermediate/execution-model-scope-and-closures/A precise model of execution contexts and scope chains, exactly what hoisting does for each declaration type, and closures used deliberately for encapsulation and memoization.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/objects-prototypes-classes-and-this/https://js.stuartsm.com/learn/intermediate/objects-prototypes-classes-and-this/Property descriptors and getters/setters, how the prototype chain implements delegation, classes as syntax over that same mechanism, and how this is actually determined.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/functional-techniques-and-collections/https://js.stuartsm.com/learn/intermediate/functional-techniques-and-collections/Composing small functions into pipelines, treating immutability as a deliberate design choice, choosing the right collection type, and writing a custom iterable.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/async-javascript-in-depth/https://js.stuartsm.com/learn/intermediate/async-javascript-in-depth/A precise event-loop mental model — the call stack, microtasks, and macrotasks — plus cancellation, retries, race conditions, and async iteration.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/browser-application-architecture/https://js.stuartsm.com/learn/intermediate/browser-application-architecture/Event delegation and custom events for scalable UI wiring, client-side navigation with the History API, web components in moderation, and the browser's core security model.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/networking-storage-and-workers/https://js.stuartsm.com/learn/intermediate/networking-storage-and-workers/HTTP methods and caching semantics, choosing among cookies/Web Storage/IndexedDB, offline strategy with service workers, and offloading CPU-bound work to a web worker.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/node-and-cross-runtime-javascript/https://js.stuartsm.com/learn/intermediate/node-and-cross-runtime-javascript/Node's core APIs — the file system, streams, processes — the two common async API styles, a minimal HTTP server, and what genuinely differs between JavaScript runtimes.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/testing-and-maintainability/https://js.stuartsm.com/learn/intermediate/testing-and-maintainability/The test pyramid as a heuristic, test doubles and when a fake beats a mock, keeping tests deterministic, and designing code around cohesion and dependency inversion so it stays testable.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/types-documentation-and-package-design/https://js.stuartsm.com/learn/intermediate/types-documentation-and-package-design/JSDoc as editor-visible contracts, TypeScript as an optional neighboring path, and designing a package's public surface — exports, versioning, and documentation.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/intermediate/performance-and-diagnostics/https://js.stuartsm.com/learn/intermediate/performance-and-diagnostics/Measuring before optimizing, recognizing algorithmic complexity in everyday code, throttling/debouncing expensive work, and finding the memory-leak sources unique to long-lived JavaScript apps.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/ecmascript-specification-literacy/https://js.stuartsm.com/learn/advanced/ecmascript-specification-literacy/How a proposal becomes a language feature, how to read specification-style algorithm steps, and using internal slots to explain apparent language oddities precisely.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/meta-programming-and-proxies/https://js.stuartsm.com/learn/advanced/meta-programming-and-proxies/Property descriptors and integrity levels precisely, symbols as collision-free keys, and Proxy/Reflect for intercepting fundamental object operations — with real observability and performance costs.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/advanced-functions-and-evaluation/https://js.stuartsm.com/learn/advanced/advanced-functions-and-evaluation/Environment retention in long-lived closures, generator delegation and coroutine-like patterns, tail-call semantics versus real engine support, and the genuine risks of dynamic evaluation.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/concurrency-scheduling-and-distributed-effects/https://js.stuartsm.com/learn/advanced/concurrency-scheduling-and-distributed-effects/How Node's event loop phases differ from the browser's, backpressure and scheduling fairness, true shared memory with SharedArrayBuffer, structured concurrency, and the realities of distributed systems.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/modules-packages-and-supply-chain/https://js.stuartsm.com/learn/advanced/modules-packages-and-supply-chain/Live bindings and how module cycles actually resolve, the dual-package hazard conditional exports can create, designing for library API stability, and dependency supply-chain risk.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/engine-behavior-and-performance-engineering/https://js.stuartsm.com/learn/advanced/engine-behavior-and-performance-engineering/Hidden classes and inline caches as engine-dependent optimizations, generational garbage collection, designing a benchmark that isn't lying to you, and when WebAssembly is (and isn't) the right boundary to reach for.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/security-and-isolation/https://js.stuartsm.com/learn/advanced/security-and-isolation/Threat modeling a JavaScript application, prototype pollution and other injection classes beyond DOM XSS, Trusted Types and cross-origin isolation, Node-specific risk, and capability-based design.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/architecture-for-large-systems/https://js.stuartsm.com/learn/advanced/architecture-for-large-systems/Ports and adapters for isolating core logic from I/O, event sourcing/CQRS as a consistency trade-off, observability as a first-class concern, and writing an ADR-driven incremental migration plan.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/advanced-testing-and-verification/https://js.stuartsm.com/learn/advanced/advanced-testing-and-verification/Property-based, mutation, fuzz, and differential testing as complements to example-based tests, hermetic builds for reproducibility, and static analysis via codemods.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/learn/advanced/api-and-language-tool-design/https://js.stuartsm.com/learn/advanced/api-and-language-tool-design/The parse/transform/generate pipeline behind every codemod and linter, designing an API's error model and diagnostics to be genuinely actionable, source maps, and plugin systems with real isolation boundaries.Thu, 03 Sep 2026 00:00:00 GMTLessonhttps://js.stuartsm.com/reference/browser/abort-controller/https://js.stuartsm.com/reference/browser/abort-controller/Cancels an in-flight fetch (or other supporting async operation) via a shared signal, rejecting its promise with an AbortError.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/events/https://js.stuartsm.com/reference/browser/events/Runs a function in response to a browser event, receiving an event object describing what happened and how to control the default behavior.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/api-design-for-libraries/https://js.stuartsm.com/reference/patterns/api-design-for-libraries/Designing a chainable fluent API that defers work to a clear terminal call, actionable diagnostics, and source maps that keep debugging transformed code usable.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/architectural-decision-records/https://js.stuartsm.com/reference/patterns/architectural-decision-records/A short, durable document capturing one significant architectural decision's context, choice, and consequences, written at the time it's made.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/arrange-act-assert/https://js.stuartsm.com/reference/patterns/arrange-act-assert/A structure for writing a clear unit test — set up the input, run the code under test, check the result.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/array-methods/https://js.stuartsm.com/reference/built-ins/array-methods/The everyday transformation and search methods on Array.prototype — which mutate, which don't, and when to reach for each.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/array-reduce/https://js.stuartsm.com/reference/built-ins/array-reduce/Combines every array element into a single accumulated result, using a callback and a starting value.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/ast-fundamentals/https://js.stuartsm.com/reference/patterns/ast-fundamentals/Every source-to-source tool follows the same pipeline — parse code into a structured Abstract Syntax Tree, transform it, generate code back out.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/async-await/https://js.stuartsm.com/reference/syntax/async-await/Syntax for writing promise-based code that reads like ordinary sequential code — await pauses only the async function it's inside.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/benchmark-design/https://js.stuartsm.com/reference/patterns/benchmark-design/Warm-up, representative input, variance reporting, and guarding against dead-code elimination — what separates a trustworthy benchmark from a misleading number.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/big-o-notation/https://js.stuartsm.com/reference/patterns/big-o-notation/A way to describe how a piece of code's cost grows as its input grows — most usefully spotted as a loop nested inside another loop over related data.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/callbacks/https://js.stuartsm.com/reference/patterns/callbacks/A function passed as an argument to another function, to be called later — the pattern behind array methods, events, and async code.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/classes/https://js.stuartsm.com/reference/syntax/classes/Syntax over the prototype mechanism — public and private fields, methods on the shared prototype, and extends for inheritance.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/closures-in-depth/https://js.stuartsm.com/reference/patterns/closures-in-depth/Using a closure deliberately to create private state — a factory function returning an object whose internal variables can't be reached from outside.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/codemods/https://js.stuartsm.com/reference/patterns/codemods/A program that transforms source code programmatically via its AST — a scripted, repeatable refactor applied across an entire codebase.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/commonjs-vs-esm/https://js.stuartsm.com/reference/patterns/commonjs-vs-esm/Node's two module systems — require/module.exports (CommonJS) and import/export (ES modules) — and how Node decides which one a file uses.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/console/https://js.stuartsm.com/reference/browser/console/A host API for writing diagnostic output — not part of the JavaScript language itself, but available in both browsers and Node.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/const-and-let/https://js.stuartsm.com/reference/syntax/const-and-let/The two modern ways to declare a variable binding — const cannot be reassigned, let can.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/custom-events/https://js.stuartsm.com/reference/browser/custom-events/Dispatches an application-defined event, carrying arbitrary data via .detail, decoupling the code that announces something from whatever reacts to it.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/date/https://js.stuartsm.com/reference/built-ins/date/Creating and reading a Date correctly, a well-known month-indexing and time-zone-parsing pitfall, and formatting dates for a human reader.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/dependency-inversion/https://js.stuartsm.com/reference/patterns/dependency-inversion/Depending on a passed-in interface rather than a concrete, hard-coded implementation — what makes substituting a test double possible.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/destructuring-and-spread/https://js.stuartsm.com/reference/syntax/destructuring-and-spread/Three related bits of syntax built on ... and pattern-matching — pulling values out, copying/combining data, and gathering leftovers.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/dynamic-evaluation-risks/https://js.stuartsm.com/reference/patterns/dynamic-evaluation-risks/eval and new Function execute a string as code with full access to their reachable scope — neither provides a meaningful sandbox boundary.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/equality-operators/https://js.stuartsm.com/reference/syntax/equality-operators/Strict equality (===) never coerces types; loose equality (==) coerces one side according to a specific set of rules.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/event-delegation/https://js.stuartsm.com/reference/patterns/event-delegation/Attaching one listener to a stable ancestor, using event bubbling to handle events from any current or future descendant.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/fetch/https://js.stuartsm.com/reference/browser/fetch/Requests a URL and returns a promise for the response — rejects only on network failure, not on an HTTP error status.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/async-iteration/https://js.stuartsm.com/reference/syntax/async-iteration/Iterates an async iterable — one whose Symbol.asyncIterator produces promises — the async counterpart to for...of.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/for-and-while-loops/https://js.stuartsm.com/reference/syntax/for-and-while-loops/Three loop forms — for when counting or indexing, while when the iteration count isn't known up front, for...of when iterating a collection's values.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/forms-and-validation/https://js.stuartsm.com/reference/browser/forms-and-validation/HTML's built-in form validation attributes, and how to read validity state or add an accessible custom error message alongside it.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/node/filesystem-and-paths/https://js.stuartsm.com/reference/node/filesystem-and-paths/Reading and writing files with Node's promise-based file system API, and building paths safely with the path module.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/function-composition/https://js.stuartsm.com/reference/patterns/function-composition/Chaining small, single-purpose functions into a pipeline, each handling one transformation.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/functions/https://js.stuartsm.com/reference/syntax/functions/Three syntaxes for creating a function value, differing in hoisting behavior and how they handle this.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/garbage-collection-generations/https://js.stuartsm.com/reference/patterns/garbage-collection-generations/Most modern engines organize the heap by object age, since most objects die young — collecting the young generation frequently and cheaply, the old generation less often.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/generator-delegation/https://js.stuartsm.com/reference/patterns/generator-delegation/Delegates part of a generator's iteration to another iterable, forwarding values and the delegate's return value transparently.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/generators-and-iterators/https://js.stuartsm.com/reference/syntax/generators-and-iterators/The Symbol.iterator protocol that powers for...of and spread, and generator functions (function*) as the easiest way to implement it.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/guard-clauses/https://js.stuartsm.com/reference/patterns/guard-clauses/Handling an edge case with an early return at the top of a function, instead of nesting the normal-case logic inside a conditional.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/hidden-classes-and-inline-caches/https://js.stuartsm.com/reference/patterns/hidden-classes-and-inline-caches/Engine-dependent optimizations that track an object's shape and cache property-access paths — consistent object shapes keep them effective.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/hoisting/https://js.stuartsm.com/reference/syntax/hoisting/How each declaration type — var, function, let, const, class — is set up before line-by-line execution begins, and what's accessible before its own line runs.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/http-caching/https://js.stuartsm.com/reference/patterns/http-caching/How the Cache-Control header controls whether and how long a response can be reused without asking the server again.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/if-else-and-switch/https://js.stuartsm.com/reference/syntax/if-else-and-switch/Branching statements — if/else for arbitrary conditions, switch for one value compared against several exact matches.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/import-export/https://js.stuartsm.com/reference/syntax/import-export/Module syntax for sharing code between files — any number of named exports, at most one default export per module.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/indexeddb/https://js.stuartsm.com/reference/browser/indexeddb/A browser API for storing large amounts of structured, queryable data client-side, well suited to offline-capable applications.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/integrity-levels/https://js.stuartsm.com/reference/patterns/integrity-levels/Three levels of object mutability restriction, each blocking a specific, increasing set of operations — freeze is shallow.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/jsdoc-annotations/https://js.stuartsm.com/reference/patterns/jsdoc-annotations/Comment-based type annotations that give editor tooling a function's real contract, with no build step required.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/json/https://js.stuartsm.com/reference/built-ins/json/Convert between a JavaScript value and its JSON text representation — the standard way data crosses storage and network boundaries.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/live-bindings/https://js.stuartsm.com/reference/patterns/live-bindings/An ES module import is a continuously updated reference, not a copied value — the mechanism that makes many circular imports resolvable.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/local-storage/https://js.stuartsm.com/reference/browser/local-storage/Persists small amounts of string data in the browser between page loads, scoped to the page's origin.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/map-and-set/https://js.stuartsm.com/reference/built-ins/map-and-set/Dedicated collection types — Map/Set for general keyed/unique-value storage, WeakMap/WeakSet for weakly-held object keys that don't prevent garbage collection.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/math-and-number-formatting/https://js.stuartsm.com/reference/built-ins/math-and-number-formatting/The Math object's standard numeric operations, and Intl.NumberFormat for locale-correct display formatting instead of hand-built strings.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/memoization/https://js.stuartsm.com/reference/patterns/memoization/Caching a pure function's results, keyed by its arguments, so a repeated call with the same input returns instantly instead of recomputing.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/memory-leak-sources/https://js.stuartsm.com/reference/patterns/memory-leak-sources/Four classic causes of a JavaScript memory leak — forgotten listeners, uncleared timers, long-lived closures, and detached DOM nodes still referenced.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/mutation-testing/https://js.stuartsm.com/reference/patterns/mutation-testing/Introduces small bugs into code and reruns the test suite, checking whether tests actually catch them — measuring test quality beyond what coverage can show.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/node/http-server/https://js.stuartsm.com/reference/node/http-server/Node's built-in HTTP server module — request/response handling with no framework, and why every input from it is untrusted.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/node-event-loop-phases/https://js.stuartsm.com/reference/patterns/node-event-loop-phases/Node's event loop cycles through distinct phases (timers, poll, check, and others), each handling a specific callback category — microtasks still drain between every phase.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/number-edge-cases/https://js.stuartsm.com/reference/built-ins/number-edge-cases/JavaScript's single numeric type is 64-bit floating point, which produces predictable rounding error and two special non-finite values.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/observability-basics/https://js.stuartsm.com/reference/patterns/observability-basics/Understanding a system's internal state from its external outputs — structured logs and a correlation id threaded through a request's full call chain.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/optional-chaining-and-nullish-coalescing/https://js.stuartsm.com/reference/syntax/optional-chaining-and-nullish-coalescing/?. reads a property and stops safely if anything along the way is missing; ?? supplies a fallback only for null or undefined.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/node/package-json/https://js.stuartsm.com/reference/node/package-json/package.json describes a Node project's scripts and dependencies; package-lock.json pins the exact versions actually installed, for reproducibility.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/node/package-exports/https://js.stuartsm.com/reference/node/package-exports/Explicitly declares a package's public entry points — anything not listed isn't importable by consumers, making internal refactoring safe.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/plugin-systems/https://js.stuartsm.com/reference/patterns/plugin-systems/Letting third-party code extend a tool's behavior, deliberately, through only the specific capabilities a plugin's job actually requires.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/ports-and-adapters/https://js.stuartsm.com/reference/patterns/ports-and-adapters/Isolates core logic behind interfaces (ports) it depends on, with concrete I/O implementations (adapters) plugged in from outside.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/promise/https://js.stuartsm.com/reference/built-ins/promise/A value representing an operation that hasn't completed yet, eventually settling as fulfilled (success) or rejected (failure).Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/tail-calls/https://js.stuartsm.com/reference/patterns/tail-calls/A specified stack-frame-reuse optimization for tail-position calls, implemented by Safari's JavaScriptCore but not V8 or SpiderMonkey — not reliable for stack safety in practice.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/property-based-testing/https://js.stuartsm.com/reference/patterns/property-based-testing/States an invariant that should hold for any valid input, and generates many varied inputs (including edge cases) to check it, shrinking failures to a minimal reproducer.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/prototype-pollution/https://js.stuartsm.com/reference/patterns/prototype-pollution/Attacker-controlled data used to set a property on Object.prototype itself, affecting every plain object in the program — distinct from DOM XSS.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/prototypes-and-object-create/https://js.stuartsm.com/reference/patterns/prototypes-and-object-create/Every object delegates to another object via its prototype; Object.create sets that link up explicitly.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/proxy-and-reflect/https://js.stuartsm.com/reference/patterns/proxy-and-reflect/Proxy intercepts fundamental object operations through traps; Reflect provides each operation's default implementation for a trap to delegate to.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/reading-spec-algorithms/https://js.stuartsm.com/reference/patterns/reading-spec-algorithms/How to read the ECMAScript specification's numbered-step, abstract-operation style, including completion records and the ? abrupt-completion shorthand.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/errors/reference-error/https://js.stuartsm.com/reference/errors/reference-error/Thrown when code refers to a binding that doesn't exist — the code was valid JavaScript, but the name it looked for wasn't there when it ran.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/regular-expressions-intro/https://js.stuartsm.com/reference/patterns/regular-expressions-intro/What problem a regular expression solves — matching a pattern of text, not one exact value — without full pattern-writing fluency yet.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/retry-with-backoff/https://js.stuartsm.com/reference/patterns/retry-with-backoff/Retrying a failed, idempotent operation with an increasing delay between attempts, to avoid hammering a struggling server.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/cors-and-csp/https://js.stuartsm.com/reference/browser/cors-and-csp/The browser's default restriction on reading cross-origin responses, how a server relaxes it with CORS, and how CSP restricts what a page can load.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/dom-selecting-and-updating/https://js.stuartsm.com/reference/browser/dom-selecting-and-updating/querySelector/querySelectorAll to find elements, and textContent/classList/setAttribute to read and update them safely.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/semantic-versioning/https://js.stuartsm.com/reference/patterns/semantic-versioning/A version-numbering convention (major.minor.patch) that promises what kind of change a release contains, based on its effect on consumers.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/service-workers/https://js.stuartsm.com/reference/browser/service-workers/A script running independently of any page that intercepts network requests, enabling deliberate offline strategies.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/shared-array-buffer/https://js.stuartsm.com/reference/browser/shared-array-buffer/Genuine shared memory between workers, with Atomics providing race-free operations on it — requires cross-origin isolation headers.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/statements-and-expressions/https://js.stuartsm.com/reference/syntax/statements-and-expressions/An expression produces a value; a statement is a complete instruction that may contain one but isn't one itself.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/node/streams/https://js.stuartsm.com/reference/node/streams/Processes data in chunks as it arrives, instead of loading an entire file or response into memory at once.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/strict-mode/https://js.stuartsm.com/reference/syntax/strict-mode/An opt-in restricted variant of JavaScript that turns several silent mistakes into thrown errors.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/string-methods/https://js.stuartsm.com/reference/built-ins/string-methods/The everyday methods for searching, slicing, and transforming text — every one returns a new string, since strings are immutable.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/structured-concurrency/https://js.stuartsm.com/reference/patterns/structured-concurrency/Binding a concurrent operation's lifetime to its parent's, so cancelling the parent explicitly propagates to every child it started.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/built-ins/structured-clone/https://js.stuartsm.com/reference/built-ins/structured-clone/Deep-copies a value using the structured clone algorithm — handles plain data, Map, Set, and Date, but not functions or DOM nodes.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/supply-chain-security/https://js.stuartsm.com/reference/patterns/supply-chain-security/A dependency is a trust decision beyond its current code — provenance, transitive dependency count, update risk, and install-time script execution all matter.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/symbols/https://js.stuartsm.com/reference/syntax/symbols/A guaranteed-unique primitive type, used as a collision-free object key and to hook user objects into built-in language behavior via well-known symbols.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/errors/syntax-error/https://js.stuartsm.com/reference/errors/syntax-error/Thrown when code isn't valid JavaScript at all — the engine refuses to run any of the file, not just the broken line.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/template-literals/https://js.stuartsm.com/reference/syntax/template-literals/Backtick-delimited strings that support embedded expressions and multi-line text without manual concatenation.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/test-doubles/https://js.stuartsm.com/reference/patterns/test-doubles/Three kinds of substitute for a real dependency in a test — a stub returns data, a mock asserts on calls, a fake is a real simplified implementation.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/dual-package-hazard/https://js.stuartsm.com/reference/patterns/dual-package-hazard/Shipping both a CommonJS and ESM build risks an app loading two separate instances of the same package, with two separate copies of any module-level state.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/event-loop/https://js.stuartsm.com/reference/patterns/event-loop/The call stack, and the two queues — microtasks and macrotasks — the event loop drains, always fully emptying microtasks before touching the next macrotask.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/history-api/https://js.stuartsm.com/reference/browser/history-api/Changes the URL shown in the address bar without a page reload, and handles the browser's back/forward buttons — the mechanism client-side routers are built on.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/tc39-process/https://js.stuartsm.com/reference/patterns/tc39-process/How a proposal advances through five stages (0-4) toward becoming a standardized ECMAScript feature, and the compatibility responsibility each stage implies.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/test-pyramid/https://js.stuartsm.com/reference/patterns/test-pyramid/A heuristic about cost and confidence — more unit tests, fewer integration tests, fewer still end-to-end tests — not a strict ratio to enforce.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/this-call-apply-bind/https://js.stuartsm.com/reference/syntax/this-call-apply-bind/this is determined by a function's call site, not where it's defined; call/apply/bind set it explicitly, and arrow functions have no this of their own.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/threat-modeling/https://js.stuartsm.com/reference/patterns/threat-modeling/Identifying a system's assets, actors, and trust boundaries — even an informal version done deliberately catches bug classes a functional review misses.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/throttle-and-debounce/https://js.stuartsm.com/reference/patterns/throttle-and-debounce/Two techniques for limiting how often expensive work runs in response to a high-frequency event — throttle caps the rate, debounce waits for a pause.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/patterns/error-handling/https://js.stuartsm.com/reference/patterns/error-handling/Validate input where it enters your code and throw a descriptive Error, so bad values can't silently propagate deeper into the program.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/trusted-types/https://js.stuartsm.com/reference/browser/trusted-types/A CSP-enforced browser API making a raw-string assignment to a DOM-XSS-sensitive sink a structural TypeError, unless it passes through a reviewed policy.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/syntax/try-catch-finally/https://js.stuartsm.com/reference/syntax/try-catch-finally/Runs a block and, if it throws, catches the error instead of crashing the program; finally always runs afterward.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/errors/type-error/https://js.stuartsm.com/reference/errors/type-error/Thrown when an operation is attempted on a value that doesn't support it — reading a property of null, calling something that isn't a function.Thu, 03 Sep 2026 00:00:00 GMTReferencehttps://js.stuartsm.com/reference/browser/web-workers/https://js.stuartsm.com/reference/browser/web-workers/Runs JavaScript on a separate thread for CPU-bound work, communicating with the main thread only via message passing.Thu, 03 Sep 2026 00:00:00 GMTReference