Tooling & Compiler

The TypeScript compiler tsc transpiles .ts files into .js. You configure it with a tsconfig.json whose compilerOptions set target, module, outDir, and strict. Running tsc reports type errors at compile time and emits JavaScript with all types erased.

Setting "strict": true in tsconfig.json turns on a family of strict type-checking flags at once, including strictNullChecks (so null and undefined are not silently assignable to other types) and noImplicitAny (an error on any inferred any). It maximizes type safety and is recommended for new projects.

target picks the ECMAScript version tsc emits. Syntax newer than the target is downleveled into equivalent older syntax; syntax the target already understands is emitted as is. It also selects the default lib declarations, but it never polyfills a missing runtime API.

sourceMap: true makes tsc emit a .js.map beside every .js, mapping each emitted position back to the original .ts line and column. Debuggers and stack traces then point at your TypeScript source instead of the compiled output. The map is a separate build artifact and changes nothing at runtime.

declaration: true makes tsc write a .d.ts beside every emitted .js, carrying the public type signatures with all implementation bodies stripped. A package ships only JavaScript, so without those files a consumer sees any; the types field in package.json points at the entry .d.ts.

declare introduces an ambient declaration — it tells the compiler about a value, type, function, or module that exists elsewhere (a global, a plain-JS library, a script-injected variable) without providing an implementation. It is type-only and emits no JavaScript. Declaration files (.d.ts) collect these ambient declarations to describe the shape of code the compiler cannot see, such as untyped libraries.

noImplicitAny is one member of the strict family: it errors wherever the compiler silently falls back to any, as on an unannotated parameter. strict: true switches the whole family on at once, and an explicitly set member overrides the umbrella.

skipLibCheck: true stops tsc from type-checking the contents of .d.ts files — your own and everything under node_modules. That cuts build time and silences errors from third-party declarations you cannot fix. In exchange, a real mistake inside a declaration goes unreported; your .ts source is still checked.

Without it, null and undefined are assignable to every type, so a string can secretly hold one and dereferencing crashes at runtime. Enabling it removes both from all other types: they must appear in an explicit union like string | null, forcing a narrowing check.

Indexing an array or an index-signature type can miss, yet by default arr[i] is typed plain T and may be dereferenced right away. The flag adds undefined to every indexed access, so the type becomes T | undefined and a check comes first. It is opt-in, not part of strict.

Author the declarations by hand and either ship them inside the package with the types field in package.json, or publish them separately as @types/<name>, which the compiler resolves automatically. Nothing verifies a .d.ts against the real JavaScript, so drift from it is silent.