29 — Declaration files and module resolution

One line: tsc checks your code against declarations it takes on trust and files it finds by its own rules, and either can disagree with what Node actually loads.

A TypeScript program is checked against types that come from somewhere: your files, .d.ts declaration files, packages and @types. The chapter starts with declaration files and the declare keyword, then how tsc finds a package's types, including TypeScript 6's change to the types default. Type-only imports and module resolution follow, the two places where tsc and Node can read the same import line differently. It ends with declaration merging, which adds to types that already exist.

The pages

Page	Level	In one line	Status
Declaration files	201	A .d.ts file holds types and no code, and tsc takes it on trust: declare const APP_VERSION: string makes APP_VERSION.toUpperCase() type-check, while Node, which never reads the file, throws ReferenceError on that line.	stub
Types for packages	201	An import finds @types/<name> by itself, but globals such as process come only from packages listed in types — and since TypeScript 6 that list starts empty, so an installed @types/node adds nothing until types names it.	stub
Type-only imports	201	Node needs every name in a plain import { ... } to exist at run time, so importing an interface without type passes tsc and then fails in Node with SyntaxError: ... does not provide an export named 'Shape'; verbatimModuleSyntax makes tsc catch it first.	stub
Module resolution	301	When Node runs the .ts file itself, import "./util.ts" is the only spelling that works: bundler accepts "./util" and nodenext accepts "./util.js", and Node refuses both with ERR_MODULE_NOT_FOUND, because it never adds or swaps an extension.	stub
Declaration merging	301	Two interface User declarations merge into one type that needs both fields, while two type aliases with one name are an error; merging into the global Array interface adds a type, not code — [1, 2, 3].last() type-checks, then throws TypeError.	stub

Boundaries

How Node itself loads modules — package.json type and exports, CommonJS interop and cycles — is in Modules.