What do skipLibCheck, declaration, and sourceMap do?

• skipLibCheck (default false): skip type-checking every declaration file, both those in node_modules and your own .d.ts. Your code is still checked against those types; only errors inside the .d.ts are not reported. It saves time and hides broken or conflicting third-party typings (two copies of one library's types, for example). The TSConfig reference tags it Recommended, and for applications I turn it on. For a library, the generated .d.ts is what consumers load, so I would keep it off at least where I build and validate them.
• declaration: emit a .d.ts beside each JS file, describing the public API. A published package needs it; composite turns it on. emitDeclarationOnly skips the JS, for when another tool produces it. declarationDir moves the .d.ts files elsewhere.
• declarationMap: emit .d.ts.map so "Go to Definition" lands in the .ts source instead of the .d.ts. The reference suggests it when you use project references.
• sourceMap: emit .js.map and append //# sourceMappingURL=... to the JS, so debuggers and error reports show TypeScript lines. Node ignores maps unless started with --enable-source-maps. inlineSources embeds the original source in the map; inlineSourceMap puts the map in the JS file and excludes sourceMap.
• noEmit: write nothing. Use it when a bundler, esbuild, or Node's type stripping produces the JavaScript and tsc is only the type-checker in CI.

Emit flags do not depend on errors: noEmitOnError defaults to false, so output is written even when type errors are reported.

// TypeScript 7.0.2 (the 5.9.3 greet.d.ts.map was byte-identical)
// tsconfig: { "compilerOptions": { "outDir": "out", "rootDir": "src", "declaration": true, "declarationMap": true, "sourceMap": true, ... } }
// src/greet.ts:  line 2:  if (name === "") throw new Error("empty name");

// tsc -p .   → out/greet.js  out/greet.js.map  out/greet.d.ts  out/greet.d.ts.map  (and the same for main)
// out/greet.d.ts
//   export declare function greet(name: string): string;
//   //# sourceMappingURL=greet.d.ts.map

// node out/main.js
// →   throw new Error("empty name");        (points into out/greet.js)
// node --enable-source-maps out/main.js
// → .../src/greet.ts:2                       (points into the .ts file)
// →   if (name === "") throw new Error("empty name");

// tsc -p . --noEmit            → no out/ directory
// tsc -p . --emitDeclarationOnly → only the .d.ts and .d.ts.map files

• Treating skipLibCheck as harmless. With a broken node_modules/bad-types/index.d.ts and a broken own src/own.d.ts, tsc printed TS2304 for both on 5.9.3, 6.0.3 and 7.0.2; with --skipLibCheck it printed nothing, so your own .d.ts mistakes also disappear.
• Setting sourceMap and expecting Node stack traces to change. Without --enable-source-maps the trace still pointed at out/greet.js.
• Using noEmit and then looking for declarations. declaration: true with noEmit wrote no files; use emitDeclarationOnly to emit only .d.ts.
• Shipping source maps without sources. sourceMap alone references ../src/greet.ts by path; add inlineSources (the map then has sourcesContent) when the original files will not be published.