types-and-interfaces

Types and Interfaces → common/types/

The folder groups type-level declarations regardless of keyword. Pick the keyword by fit, not folder:

• interface for object shapes (extends and merges cleanly)
• type for what an interface can't express (unions, intersections, mapped types, primitives, tuples, function signatures)
• Either works for an object shape → stay consistent within a domain. Refactoring between the keywords is an in-place edit; the filename and imports never change.

A discriminated union family lives in types/ under the union's name.

The Params interface stays with its function, and a type may stay with the single value typed by it (one-export-per-file's exception 5); every other exported type goes in types/:

// copyFile.ts — Params co-located, unexported
interface Params {
	sourcePath: string;
	destPath: string;
}

export const copyFile = ({ sourcePath, destPath }: Params) => { /* ... */ };

// common/types/CopyResult.ts — exported return type gets its own types/ file
export interface CopyResult {
	success: boolean;
	bytesWritten: number;
}