Declaration File Generation

Declaration files (.d.ts) describe the public type interface of compiled JavaScript. They are important for typed library consumers but do not contain executable implementations. Set declaration:true when distributing a TypeScript library, and ensure published package entry points actually reference the generated files.

The implementation runs normally. Compiling with --declaration generates a .d.ts signature containing Contact and label, while the emitted .js still performs the string formatting.

A declaration file alone cannot supply runtime functions; consumers also need the matching JavaScript implementation.

Save as declaration-file-generation.ts. The following program passed strict TypeScript compilation, and the displayed output was captured by executing its emitted JavaScript with Node.js 22.16.0.

TypeScript source

export interface Contact { id: number; name: string }
export function label(contact: Contact): string { return `${contact.id}: ${contact.name}`; }
console.log(label({ id: 2, name: "Asha" }));

Verified output

2: Asha

Compile: npx tsc --strict --target es2022 --module commonjs declaration-file-generation.ts; run: node declaration-file-generation.js. For Node.js API examples, install @types/node first. For browser projects, use the browser-specific instructions below.

The example demonstrates the code-level behavior; the related configuration or test commands are shown separately when needed.

The implementation runs normally. Compiling with --declaration generates a .d.ts signature containing Contact and label, while the emitted .js still performs the string formatting.

Predict what changes if you alter one input. Then run the code to compare your reasoning with the real output.

Practical project lab: These are real configuration and source-file examples, not a TypeScript object that merely describes settings. Place the files at the named paths relative to one project directory.

tsconfig.json

{"compilerOptions":{"target":"ES2022","module":"CommonJS","strict":true,"declaration":true}}

math.ts

export function square(n: number): number { return n * n; }

Commands

npx tsc -p .

Read math.d.ts and math.js: one declares the API; the other implements it.

This local compiler or test-runner mini-project was validated in addition to the primary program.

A declaration file alone cannot supply runtime functions; consumers also need the matching JavaScript implementation.

Identify whether the failure is a type-checking issue, a configuration issue, or a runtime behavior issue before choosing a fix.

Run tsc with --declaration and compare the generated JS and d.ts contents.

Write down your expected result first. For configuration examples, compare compiler output and diagnostics; for test examples, make the assertion fail deliberately to verify that the tool reports it.

For the exact current options and platform behavior, consult the official documentation. These examples were verified using the tool versions described above.

Declaration files (.

Run the primary example, then complete the practice task or environment-specific lab.