How to set types for absolute path and relative path?

Question

I need to deal with absolute path variables and relative path variables in Deno TypeScript. The std library has the ParsedPath type, but that's not specific enough. Besides adding the absolute/relative word in the variable name, is there a way to type it as well? At best it should work in both Windows and Linux, but if it's hard then Windows is suffice.

Answer 1

First, some observations:

• The properties of the ParsedPath interface are mutable, so whatever type you derive from it must enforce that instances cannot be mutated (e.g. by using the utility Readonly<Type>) — otherwise, a consumer might make changes to it, which would circumvent the type system discrimination that you seek.

• The string property root will be the empty string "" for relative parsed paths — which can be used to discriminate whether the path is relative or absolute.

Here is an example:

Code in TS Playground

import { parse } from "https://deno.land/[email protected]/path/parse.ts";
import type { ParsedPath } from "https://deno.land/[email protected]/path/_interface.ts";

type AbsoluteParsedPath = Readonly<ParsedPath> & { readonly isAbsolute: true };
type RelativeParsedPath = Readonly<ParsedPath> & { readonly isAbsolute: false };

function parsePath(path: string): AbsoluteParsedPath | RelativeParsedPath {
  const parsed = parse(path);
  return Object.assign(parsed, { isAbsolute: parsed.root.length > 0 });
}

declare const inputPath: string;

const parsed = parsePath(inputPath);
//    ^? const parsed: AbsoluteParsedPath | RelativeParsedPath

if (parsed.isAbsolute) {
  parsed
  //^? const parsed: AbsoluteParsedPath
} else {
  parsed
  //^? const parsed: RelativeParsedPath
}