Explicit dependencies
Organize application code into files with explicit imports and exports.
TypeScript · Const
module
A TypeScript file with a top-level import or export is a module; a file without either is a script. Modules have their own scope, while scripts contribute declarations to the global scope.
How it works
TypeScript treats a file with a top-level import or export as a module. That file has its own scope, so its declarations are not placed in the global scope; files without top-level imports or exports are scripts instead.
Syntax
import { name } from "module";
export { name };Key points
- A top-level
importorexportis enough to classify the file as a module. - Without a top-level import or export, the file is classified as a script.
- Module execution uses a private scope instead of the global scope.
- TypeScript uses the ECMAScript 2015 module classification rule.
Common mistakes
- A file containing no top-level import or export is a script, so its declarations are globally visible rather than module-scoped.
- Module declarations do not automatically become global names; each module executes in its own scope.
Example: A minimal TypeScript module
export const answer = 42;
import { answer } from "./answer.js";
console.log(answer);A file with a top-level export is a module, and another module can import its exported binding.
When to use it
Scope isolation
Prevent unrelated files from sharing names through the global scope.
Controlled exports
Expose only selected APIs from an implementation file.
Interview questions
QHow does TypeScript decide whether a file is a module?
A TypeScript file is a module when it contains a top-level import or export. Otherwise, TypeScript treats it as a script, whose contents are available in the global scope.
Follow-up: How do you make a file a module when it has no runtime imports or exports? Add an explicit type-only import or export, such as export {}.
QWhy are modules useful in a large codebase?
Module scope prevents declarations from being shared through the global scope. Files can expose selected bindings through exports and consume them through imports.
Follow-up: What is the practical benefit in a large codebase? Explicit dependency boundaries reduce accidental name collisions and make relationships easier for tooling to analyze.
QIs module status the same as the emitted module format?
The module concept follows ECMAScript 2015: top-level import and export determine module status. This is separate from the particular runtime module format used when JavaScript is emitted.
Follow-up: Which compiler setting addresses emitted format? The module compiler option controls the module format of emitted JavaScript and also informs related module analysis.
Related
importexportscriptmoduleNamespaces and Modules