typescript

A type earns its place by making a wrong program fail to compile. Three biases decide most calls:

• Make the invalid state unrepresentable, then let inference carry the rest. An annotation that restates what the compiler already infers is noise; a type that admits a state the domain forbids is a defect that will be found at runtime.
• Every escape hatch is a defect report against a type. any, as, !, and @ts-ignore each move a failure from the compiler to production. Reaching for one means the declaration is wrong, and that is where the fix belongs.
• The tsconfig.json decides what the compiler catches; the pinned release decides what may be written. Read both before writing code.

Compiler Release

The typescript entry in the project's package.json sets the release. Read it before writing code and never reach for a feature or an option above it. Releases below 5.5 are not indexed here; a rule naming a feature older than 5.5 carries its version where it is named.

• 5.5 — inferred type predicates; isolatedDeclarations; ${configDir}; regex literal syntax checking
• 5.6 — noUncheckedSideEffectImports; noCheck; strictBuiltinIteratorReturn; always-truthy and always-nullish checks become errors; package.json "type" inside node_modules is respected in every module mode
• 5.7 — rewriteRelativeImportExtensions; never-initialized variable checks; a JSON import into an ES module under nodenext requires with { type: "json" }; every TypedArray takes a buffer type parameter
• 5.8 — erasableSyntaxOnly; module node18; libReplacement; require() of ESM under nodenext; each branch of a conditional in a return checked separately
• 5.9 — import defer; module node20; the prescriptive tsc --init template; ArrayBuffer stops being a supertype of the TypedArray types
• 6.0 — strict, module: esnext, target: es2025, noUncheckedSideEffectImports: true, libReplacement: false, rootDir: ".", and types: [] become the defaults; outFile, moduleResolution: classic/node10, target: es5, downlevelIteration, module: amd/umd/systemjs/none, baseUrl, esModuleInterop: false, and alwaysStrict: false are deprecated, each silenceable for one release with "ignoreDeprecations": "6.0"; the module keyword for a namespace and the assert import attribute are hard errors ignoreDeprecations does not reach; stableTypeOrdering; subpath imports starting #/
• 7.0 — the compiler is a native Go binary; every 6.0 deprecation becomes a removal; the 6.0 programmatic API is gone; stableTypeOrdering is fixed on; --checkers, --builders, --singleThreaded; template-literal inference consumes whole Unicode code points

Read [${CLAUDE_SKILL_DIR}/references/versions/ts-N.N.md] — one file per release, ts-5.5.md through ts-7.0.md — when writing against a feature near the pinned release, and whenever raising it. Each carries what its release added, which existing behavior it changed, and the traps it introduced.

A 6.0 or 7.0 upgrade is a configuration change before it is a code change. Set rootDir and types explicitly before anything else. A rootDir left to the new default moves output into dist/src/ when the config sits above the sources, and reports TS5011 naming the directory it wants. types is the one that fails silently: its [] default drops every global declaration until the packages are listed, and the symptom is a burst of Cannot find name errors that say nothing about the option that caused them.