tsentials

Use when creating structured error values — Err factory methods (failure/validation/notFound/conflict/unauthorized/forbidden/unexpected/fromException/combine/equals), AppError with code/description/type/metadata, ErrorType enum, and ErrorMetadata namespace.

Use when objects need deep-copy semantics — implement Cloneable<T> on domain objects with custom clone logic, use deepClone() for any value (plain objects, Date, Map, Set, TypedArrays, Error, circular refs — never throws, React Native / Hermes compatible), and cloneArray() to call .clone() on every element in a Cloneable collection.

Use when building DDD domain models — createEntityBase() mixin factory for aggregate roots with audit fields and domain events, createSoftDeletable() for soft deletion lifecycle, and DomainEvent marker interface for event-driven design. Uses composition (private field delegation), not class inheritance.

Use when making HTTP calls that should return Result<T> instead of throwing — fetchResult const object with get/post/put/patch/delete methods, RequestBuilder for fluent header/query-param/body construction with .send<T>(), httpStatusToError for status-to-AppError mapping, extractErrorDescription for ProblemDetails parsing.

Use when representing optional values explicitly — Maybe<T> as a null-safe container with hasValue discriminant, Maybe.from()/some()/none()/fromTry() for creation, map/bind/filter/tap/tapNone chaining, match/switch for consumption, conditional mapIf/bindIf, fallback or/orElse, extraction getOrDefault/getOrElse/getOrUndefined/getOrThrow/deconstruct, flatten/toArray utilities, async support with mapAsync/bindAsync/filterAsync/tapAsync/matchAsync/orAsync, and collection utilities tryFirst/tryLast/tryFind/choose/asMaybe.

Use when deciding which tsentials module to use — overview of all 18 subpath imports organized by concern, install command, and a quick-reference table mapping problems to modules.

Use when handling operation outcomes without exceptions — Result<T> for success/failure, railway-oriented chaining with then/map/ensure, ResultChain for fluent sync pipelines, ResultAsync<T> for async chains that resolve once at the end, Result.combine for merging heterogeneous results, and maybe-bridge for Maybe<->Result conversion.

Use when composing business validation logic — define rules as typed functions returning VoidResult, combine with RuleEngine.and/or/linear/if, evaluate synchronously or asynchronously with evaluateAsync.

Master skill for tsentials — railway-oriented programming library for TypeScript. Load this skill first when working with any tsentials module. Contains critical naming rules, API patterns, decision trees for module selection, and references to all 18 module-specific skills.

Use when you need testable time — DateTimeProvider interface with utcNow()/utcNowDate()/utcNowMs() so production code uses the SystemDateTimeProvider const object while tests use createFakeDateTimeProvider to freeze, advance, or set the clock without touching Date.now().

Use when parsing or stringifying JSON safely without exceptions — safeJsonParse/safeJsonStringify return Result<T> instead of throwing, parseAndValidate combines parsing with a custom type guard, and Json/JsonObject/JsonArray/JsonPrimitive types plus isJson/isJsonObject guards enforce structural validity at compile and runtime.

Use when a value can be one of several distinct tagged types — Union<T> as a compile-time-safe discriminated union with Union.match() for exhaustive pattern matching, Union.is() for type guards, and Union.of() for construction.