Microsoft Typescript

microsoft/TypeScript typescript

TypeScript is a language for application scale JavaScript development

Version: 6.0.2 Tags: dev: 3.9.4, tag-for-publishing-older-releases: 4.1.6, insiders: 4.6.2-insiders.20220225, latest: 6.0.2, beta: 6.0.0-beta, rc: 6.0.1-rc, next: 6.0.0-dev.20260323

References: package.json — exports, entry points • README — setup, basic usage • Docs — API reference, guides • GitHub Issues — bugs, workarounds, edge cases • Releases — changelog, breaking changes, new APIs

Search

Use skilld search "query" -p typescript instead of grepping .skilld/ directories. Run skilld search --guide -p typescript for full syntax, filters, and operators.

API Changes

This section documents version-specific API changes for TypeScript v5.7+, prioritizing recent major/minor releases.

• BREAKING: ArrayBuffer no longer supertype of TypedArray types — v5.9 changed ArrayBuffer relationships; methods expecting ArrayBuffer now reject Uint8Array, Buffer, etc. Fix by accessing .buffer property or using specific Uint8Array<ArrayBuffer> types source

• NEW: import defer * as ns from "module" — v5.9 syntax for deferred module evaluation; module loading deferred until namespace accessed, useful for conditional/lazy imports. Only supports namespace imports, not named/default exports source

• BREAKING: TypedArray generics over ArrayBufferLike — v5.7+ made all TypedArray types generic (Uint8Array<TArrayBuffer>), breaking code passing Buffer/Uint8Array where ArrayBuffer expected. Update @types/node and specify explicit buffer type or use .buffer property source

• NEW: --module node20 — v5.9 stable option modeling Node.js v20 behavior; unlike nodenext, implies --target es2023 and is not floating. Use when locked to Node.js v20 source

• NEW: --erasableSyntaxOnly option — v5.8 flag for Node.js 23.6+ --experimental-strip-types mode; errors on non-erasable TypeScript constructs (enum, namespace with code, parameter properties, import =, export =) source

• NEW: --module node18 — v5.8 stable option for Node.js 18; disallows require() of ESM but allows import assertions (unlike nodenext) source

• NEW: require() of ECMAScript modules — v5.8 --module nodenext now permits require("esm") from CommonJS (Node.js 22+), except for ESM with top-level await source

• BREAKING: Import assertions deprecated in --module nodenext — v5.8 rejects assert { type: "json" } syntax in favor of with { type: "json" } source

• NEW: --rewriteRelativeImportExtensions option — v5.7 compiler option rewrites relative .ts imports to .js when emitting, enabling in-place execution then compilation source

• NEW: --target es2024 and --lib es2024 — v5.7 support for ES2024 features (Object.groupBy, Map.groupBy, Promise.withResolvers, SharedArrayBuffer types) source

• NEW: Granular return expression checking — v5.8 type-checks each branch of conditional return statements against declared return type, catching type mismatches with any corruption source

• BREAKING: JSON import validation in --module nodenext — v5.7 requires with { type: "json" } attribute for JSON imports; no named exports, only default source

• NEW: --libReplacement flag — v5.8 option to disable/enable @typescript/lib-* package lookup; default behavior may change in future versions source

• BREAKING: Index signatures from non-literal class methods — v5.7 now generates index signatures for non-literal computed method names (e.g., [symbolName]() {}), changing class type shape source

• NEW: Preserved computed property names in declarations — v5.8 preserves entity names (bare variables, dotted names) in computed property declarations in classes source

Also changed: Checks for never-initialized variables (v5.7) · Minimal tsc --init output (v5.9) · Type argument inference changes may introduce new errors (v5.9) · V8 compile caching in Node.js (v5.7) · More implicit any errors on functions returning null/undefined (v5.7) · Expandable hovers preview (v5.9) · Configurable hover length js/ts.hover.maximumLength (v5.9) · Search ancestor tsconfig.json files (v5.7) · Faster project ownership checks for composite projects (v5.7)

Best Practices

• Use lowercase primitive type names (string, number, boolean, symbol) rather than capitalized versions (String, Number, Boolean, Symbol), which refer to boxed objects rarely used in JavaScript code source

• Avoid any unless actively migrating JavaScript to TypeScript; prefer unknown when the input type is truly unknown, as it forces explicit type checking before use source

• Use void for callback return types when the return value will be ignored; this prevents accidental access to the return value and catches real bugs source

// Good - prevents accidental use of return value
function fn(x: () => void) {
  x(); // return value is ignored
}

• Write callback parameters as required rather than optional unless you specifically need to allow invocation with fewer arguments; optional parameters create misleading contracts about how the callback can be called source

• Order function overloads from specific signatures to general ones; TypeScript selects the first matching overload, so more specific cases must come before general ones source

• Use optional parameters instead of multiple overloads that differ only in trailing parameters; this enables better error detection in higher-order functions and respects strict null checks source

• Use union types instead of overloads when signatures differ only in the type of a single argument; union signatures enable "pass-through" patterns where values can be forwarded without losing type information source

• Avoid writing generic types that don't use their type parameter, as unused generics provide no type safety benefit and confuse readers about intent source

• Enable strict compiler options in tsconfig.json: strict, noUncheckedIndexedAccess, exactOptionalPropertyTypes, and verbatimModuleSyntax catch real bugs at compile time rather than runtime source

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "verbatimModuleSyntax": true,
    "moduleDetection": "force"
  }
}

• Rely on contextual typing to infer callback parameter types rather than explicitly annotating each one; TypeScript infers parameter types from the context in which the callback is used, reducing boilerplate source

• Use utility types like Pick, Omit, Partial, Required, and Record to transform existing types instead of manually rewriting object shapes; these compose reliably and reduce duplication source

• Provide explicit type annotations for array/collection literals when TypeScript's "best common type" algorithm fails; this is necessary when array elements have no single common base type across all candidates source

// TypeScript infers (Rhino | Elephant | Snake)[] without annotation
// Explicit annotation needed to get Animal[]
const zoo: Animal[] = [new Rhino(), new Elephant(), new Snake()]

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