feat: add npm user validity check (#659)

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## PR Checklist

- [x] Addresses an existing open issue: fixes #649 
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/tidelift-me-up/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/tidelift-me-up/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

<!-- Description of what is changed and how the code change does that.
-->

This PR adds a user validity check if the API response from the npm
registry API returns zero packages to differentiate between invalid npm
users and valid users with zero packages.

### How It Works

1. The npm registry API is queried with the npm username input.
2. If the response contains one or more packages, result is returned as
usual. (This is the "happy" case.)
3. If the response contains 0 packages, check if npm username exists
using the [npm-user wrapper](https://github.com/sindresorhus/npm-user).
4. If npm username doesn't exist, return an `Invalid npm username.`
error.
6. If npm username does exist but user has zero packages, return a `No
packages found for npm username: ${username}.` error.

### Testing

I updated the unit tests that were affected by these changes and then I
ran the unit tests using `pnpm run test`.

🤖

Author: maggienegm

package.json

@@ -39,6 +39,7 @@
 	},
 	"dependencies": {
 		"chalk": "^5.4.1",
+		"npm-user": "^6.1.1",
 		"npm-username-to-packages": "^0.1.3",
 		"npm-whoami": "^1.1.4"
 	},

pnpm-lock.yaml

@@ -0,0 +1,9 @@
+declare module "npm-user" {
+	export default function npmUser(username: string): Promise<{
+		avatar?: string;
+		email?: string;
+		github?: string;
+		name?: string;
+		twitter?: string;
+	}>;
+}

src/npm-user.d.ts

@@ -25,6 +25,14 @@ vi.mock("./getPackageEstimates.js", () => ({
 	getPackageEstimates: () => [],
 }));
 
+const { mockNpmUser } = vi.hoisted(() => {
+	return { mockNpmUser: vi.fn() };
+});
+
+vi.mock("npm-user", () => {
+	return { default: mockNpmUser };
+});
+
 describe("tideliftMeUp", () => {
 	it("throws an error when --username isn't provided and getNpmWhoami returns undefined", async () => {
 		mockGetNpmWhoami.mockResolvedValue(undefined);
@@ -39,34 +47,67 @@ describe("tideliftMeUp", () => {
 
 		mockNpmUsernameToPackages.mockResolvedValue([]);
 		mockGetNpmWhoami.mockRejectedValue(new Error("Should not be called."));
+		mockNpmUser.mockRejectedValue(new TypeError("Username required"));
 
 		await expect(() => tideliftMeUp({ username })).rejects.toThrowError(
-			`No packages found for npm username: ${username}.`,
+			`Invalid npm username: ${username}.`,
 		);
 	});
 
 	it("throws an error when user is logged in but has no packages", async () => {
 		const username = "abc123";
+		const npmUserData = {
+			avatar: "",
+			email: "",
+			github: "",
+			name: "",
+			twitter: "",
+		};
 
 		mockNpmUsernameToPackages.mockResolvedValue([]);
 		mockGetNpmWhoami.mockResolvedValue(username);
+		mockNpmUser.mockResolvedValue(npmUserData);
 
 		await expect(() => tideliftMeUp()).rejects.toThrowError(
 			`No packages found for npm username: ${username}.`,
 		);
 	});
 
-	it("throws an error when valid --username is provided and has no packages", async () => {
+	it("throws an error when provided --username is valid but has no packages", async () => {
 		const username = "abc123";
+		const npmUserData = {
+			avatar: "",
+			email: "",
+			github: "",
+			name: "",
+			twitter: "",
+		};
 
 		mockNpmUsernameToPackages.mockResolvedValue([]);
 		mockGetNpmWhoami.mockRejectedValue(new Error("Should not be called."));
+		mockNpmUser.mockResolvedValue(npmUserData);
 
 		await expect(() => tideliftMeUp({ username })).rejects.toThrowError(
 			`No packages found for npm username: ${username}.`,
 		);
 	});
 
+	it("throws an error when provided --username is valid but doesn't exist", async () => {
+		const username = "nonexistent-user";
+
+		mockNpmUsernameToPackages.mockResolvedValue([]);
+		mockGetNpmWhoami.mockRejectedValue(new Error("Should not be called."));
+		mockNpmUser.mockRejectedValue(
+			Object.assign(new Error(`User \`${username}\` could not be found`), {
+				code: "ERR_NO_NPM_USER",
+			}),
+		);
+
+		await expect(() => tideliftMeUp({ username })).rejects.toThrowError(
+			`Npm user not found: ${username}.`,
+		);
+	});
+
 	it("calls npmUserPackages with the npm name when the user is logged in and user has packages", async () => {
 		const username = "abc123";