Explain tsconfig.json code

What this tsconfig.json sample shows

compilerOptions

target is the JavaScript version to write, rootDir and outDir say where the .ts files are and where the .js files go. declaration adds a .d.ts type file next to each .js file and sourceMap a .js.map, so one npx tsc wrote six files for two sources: index.js, index.d.ts, index.js.map and the same three for prices.

strict checks

strict turns on a group of checks. One of them stopped function double(x) with error TS7006: Parameter 'x' implicitly has an 'any' type. noUncheckedIndexedAccess isn't part of strict: it treats prices[0] as maybe missing, so first.toFixed(2) failed with 'first' is possibly 'undefined'. and passed with the option off.

modules

NodeNext makes TypeScript follow Node's own rules for imports, which depend on package.json's "type": "module". Under them a relative import needs its file extension, and you write .js even though the file on disk is .ts: import { total } from "./prices" failed with error TS2835: Relative import paths need explicit file extensions, and "./prices.js" worked. Without the "type" line, the same import compiled.

include and exclude

include lists the files to start from and exclude takes some of them back out, so npx tsc --listFilesOnly named src/prices.ts and src/index.ts but not the test file. Patterns use ** for any number of folders.

Good to know

• Run with TypeScript 5.9.3 on a two-file project: npx tsc finished with no errors, and node dist/index.js printed total 19.75 and cheapest 3.25.
• Unlike package.json, a tsconfig.json may have // comments, and the compiler read this one, comment included, without complaint.
• skipLibCheck skips checking every .d.ts file, the ones in installed packages and any you write yourself (TypeScript's docs: "Skip type checking of declaration files."). Builds get faster, and errors inside those files go unreported; your .ts files are still checked.

Watch out: exclude only changes where TypeScript starts, not what it compiles. When src/run-tests.ts imported ./prices.test.js, the test file was checked and written to dist/ anyway, and --listFilesOnly listed it. To keep a file out of the build, nothing the build includes may import it.