Agent skill

Typescript Test Doubles

Test doubles with Vitest (vi.fn, vi.mock, vi.spyOn) and Mother pattern.

MITAuto-check passedTesting & QA

Install Typescript Test Doubles

skills CLI

$ npx skills add macalbert/envilder --skill typescript-test-doubles -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI

$ gh skill install macalbert/envilder typescript-test-doubles --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy

$ git clone --depth 1 https://github.com/macalbert/envilder.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.github/skills/typescript-test-doubles .claude/skills/typescript-test-doubles && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Paste into Claude Code

Install the "typescript-test-doubles" agent skill from https://github.com/macalbert/envilder/tree/main/.github/skills/typescript-test-doubles into .claude/skills/typescript-test-doubles/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "typescript-test-doubles", then confirm the skill loads.

Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

$skill-installer

$skill-installer install https://github.com/macalbert/envilder/tree/main/.github/skills/typescript-test-doubles

Type this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.

skills CLI

$ npx skills add macalbert/envilder --skill typescript-test-doubles -a codex

Project install goes to .agents/skills/; add -g for ~/.codex/skills/.

GitHub CLI

$ gh skill install macalbert/envilder typescript-test-doubles --agent codex

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/macalbert/envilder.git skills-src && mkdir -p .agents/skills && cp -r skills-src/.github/skills/typescript-test-doubles .agents/skills/typescript-test-doubles && rm -rf skills-src

Use ~/.agents/skills/ instead of .agents/skills for a personal install.

Codex skills documentation · loads skills from .agents/skills/

Paste into Codex

Install the "typescript-test-doubles" agent skill from https://github.com/macalbert/envilder/tree/main/.github/skills/typescript-test-doubles into .agents/skills/typescript-test-doubles/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "typescript-test-doubles", then confirm the skill loads.

Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add macalbert/envilder --skill typescript-test-doubles -a cursor

Project install goes to .agents/skills/; add -g for ~/.cursor/skills/.

GitHub CLI

$ gh skill install macalbert/envilder typescript-test-doubles --agent cursor

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/macalbert/envilder.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/.github/skills/typescript-test-doubles .cursor/skills/typescript-test-doubles && rm -rf skills-src

Use ~/.cursor/skills/ instead of .cursor/skills for a personal install.

Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/

Paste into Cursor

Install the "typescript-test-doubles" agent skill from https://github.com/macalbert/envilder/tree/main/.github/skills/typescript-test-doubles into .cursor/skills/typescript-test-doubles/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "typescript-test-doubles", then confirm the skill loads.

Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

gemini skills install

$ gemini skills install https://github.com/macalbert/envilder.git --path .github/skills/typescript-test-doubles

--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.

skills CLI

$ npx skills add macalbert/envilder --skill typescript-test-doubles -a gemini-cli

Project install goes to .agents/skills/; add -g for ~/.gemini/skills/.

GitHub CLI

$ gh skill install macalbert/envilder typescript-test-doubles --agent gemini-cli

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/macalbert/envilder.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/.github/skills/typescript-test-doubles .gemini/skills/typescript-test-doubles && rm -rf skills-src

Use ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.

Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/

Paste into Gemini CLI

Install the "typescript-test-doubles" agent skill from https://github.com/macalbert/envilder/tree/main/.github/skills/typescript-test-doubles into .gemini/skills/typescript-test-doubles/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "typescript-test-doubles", then confirm the skill loads.

Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

GitHub CLI

$ gh skill install macalbert/envilder typescript-test-doubles

Installs for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).

skills CLI

$ npx skills add macalbert/envilder --skill typescript-test-doubles -a github-copilot

Project install goes to .agents/skills/; add -g for ~/.copilot/skills/.

Manual copy

$ git clone --depth 1 https://github.com/macalbert/envilder.git skills-src && mkdir -p .github/skills && cp -r skills-src/.github/skills/typescript-test-doubles .github/skills/typescript-test-doubles && rm -rf skills-src

Use ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.

GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/

Paste into GitHub Copilot

Install the "typescript-test-doubles" agent skill from https://github.com/macalbert/envilder/tree/main/.github/skills/typescript-test-doubles into .github/skills/typescript-test-doubles/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "typescript-test-doubles", then confirm the skill loads.

GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add macalbert/envilder --skill typescript-test-doubles -a opencode

OpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.

GitHub CLI

$ gh skill install macalbert/envilder typescript-test-doubles --agent opencode

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/macalbert/envilder.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/.github/skills/typescript-test-doubles .opencode/skills/typescript-test-doubles && rm -rf skills-src

Use ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.

OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/

Paste into OpenCode

Install the "typescript-test-doubles" agent skill from https://github.com/macalbert/envilder/tree/main/.github/skills/typescript-test-doubles into .opencode/skills/typescript-test-doubles/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "typescript-test-doubles", then confirm the skill loads.

OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

Facts

Skill name: typescript-test-doubles
GitHub stars: 138
Token cost: ~1.2k tokens
SKILL.md length: 216 words
Files: 1
Skills in repo: 30
Repo updated: Oct 5, 2026
First seen: Oct 7, 2026
Licence: MIT

At a glance

Test doubles with Vitest (vi.fn, vi.mock, vi.spyOn) and Mother pattern.

Works in 5 steps: Mock: vi.fn() → Stub: mockResolvedValue / mockReturnValue → Spy: vi.spyOn() → …
Creating test data
SKILL.md covers Test Doubles Types, Verification Patterns, Mother Pattern and Port Test Double Pattern, plus 1 more section
Needs API_KEY

What it does

Typescript Test Doubles is an agent skill from macalbert/envilder. Test doubles with Vitest (vi.fn, vi.mock, vi.spyOn) and Mother pattern. Use when creating test data, mocking dependencies, or setting up module mocks in TypeScript tests.

Its SKILL.md is about 1.2k tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.

It sits in Testing & QA, covering Unit testing. It works with TypeScript and Vitest. The repository describes itself as: One secret mapping for local dev, CI/CD, and runtime. Envilder resolves cloud secrets from your own vaults without SaaS middlemen, duplicated config, or .env drift. The licence is MIT.

When your agent uses it

Creating test data
Mocking dependencies
Setting up module mocks in TypeScript tests

Example prompts

“/typescript-test-doubles”

Requirements

A credential in API_KEY

Workflow steps

5 steps, taken from the step headings in SKILL.md.

Mock: vi.fn()
Stub: mockResolvedValue / mockReturnValue
Spy: vi.spyOn()
Module Mock: vi.mock()
Error Simulation: mockRejectedValue

What it can do on your machine

Read from SKILL.md and the folder at commit b6a0327. It shows what the files ask for, not the result of running them.

Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md (its code samples are typescript).
From the folder's file list and the shell code blocks in SKILL.md.
Network
No URLs in SKILL.md.
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names these keys or tokens, usually read from environment variables:
- API_KEY
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Typescript Test Doubles loads about 1.2k tokens when it runs. Until then it costs about 49 tokens; SKILL.md has 216 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it: ~49
When it runs · the whole SKILL.md, loaded when a task matches: ~1.2k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from macalbert/envilder at commit b6a0327, republished under its MIT licence (© macalbert). 216 words, ~1,204 tokens.

Download SKILL.mdSave it as .claude/skills/typescript-test-doubles/SKILL.md (or your agent's skills folder).

name: typescript-test-doubles
description: Test doubles with Vitest (vi.fn, vi.mock, vi.spyOn) and Mother pattern. Use when creating test data, mocking dependencies, or setting up module mocks in TypeScript tests.

Test Doubles (TypeScript)

This skill defines how to use test doubles in TypeScript (Vitest).

Test Doubles Types

1. Mock: vi.fn()

Use vi.fn() to create mock functions for port interfaces.

Purpose: Replace real dependencies with controllable test doubles.

typescript

const mockProvider: ISecretProvider = {
  getSecret: vi.fn(),
};

const mockStore: IVariableStore = {
  getMapping: vi.fn(),
  saveEnvironment: vi.fn(),
};

const mockLogger: ILogger = {
  info: vi.fn(),
  warn: vi.fn(),
  error: vi.fn(),
};

2. Stub: mockResolvedValue / mockReturnValue

Use return value methods to configure stubs.

Purpose: Predefined responses without caring about call verification.

typescript

// Async stub
mockProvider.getSecret.mockResolvedValue('secret-value');
mockStore.getMapping.mockResolvedValue({ KEY: '/ssm/path' });

// Sync stub
mockParser.parse.mockReturnValue({ config: {}, mappings: {} });

// Sequential returns
mockProvider.getSecret
  .mockResolvedValueOnce('first')
  .mockResolvedValueOnce('second');

3. Spy: vi.spyOn()

Use vi.spyOn() to observe calls on real objects without replacing behavior.

Purpose: Verify interactions while preserving real implementation.

typescript

const spy = vi.spyOn(console, 'log');

sut.execute();

expect(spy).toHaveBeenCalledWith('Processing...');
spy.mockRestore();

4. Module Mock: vi.mock()

Use vi.mock() to replace entire modules.

Purpose: Replace external dependencies (AWS SDK, file system, etc.).

typescript

vi.mock('@aws-sdk/client-ssm', () => ({
  SSMClient: vi.fn().mockImplementation(() => ({
    send: vi.fn(),
  })),
  GetParameterCommand: vi.fn(),
}));

5. Error Simulation: mockRejectedValue

Use mockRejectedValue() to simulate failures.

Purpose: Test error paths and exception handling.

typescript

mockProvider.getSecret.mockRejectedValue(
  new ParameterNotFoundError('/missing/key'),
);

mockStore.getMapping.mockRejectedValue(
  new Error('File not found'),
);

Verification Patterns

Basic Verification

typescript

expect(mockProvider.getSecret).toHaveBeenCalledWith('/ssm/path');
expect(mockProvider.getSecret).toHaveBeenCalledTimes(1);
expect(mockLogger.info).toHaveBeenCalledWith(
  expect.stringContaining('success'),
);

Not Called

typescript

expect(mockStore.saveEnvironment).not.toHaveBeenCalled();
expect(mockLogger.error).not.toHaveBeenCalled();

Call Order

typescript

expect(mockProvider.getSecret).toHaveBeenCalledBefore(
  mockStore.saveEnvironment,
);

Argument Matchers

typescript

expect(mockLogger.info).toHaveBeenCalledWith(
  expect.stringMatching(/loaded \d+ secrets/),
);
expect(mockStore.saveEnvironment).toHaveBeenCalledWith(
  expect.objectContaining({ KEY: 'value' }),
);

Mother Pattern

Use factory functions for reusable test data creation.

typescript

function createParsedMapFile(
  overrides: Partial<ParsedMapFile> = {},
): ParsedMapFile {
  return {
    config: { provider: 'aws' },
    mappings: new Map([['DB_URL', '/app/db']]),
    ...overrides,
  };
}

function createEnvilderOptions(
  overrides: Partial<EnvilderOptions> = {},
): EnvilderOptions {
  return {
    provider: SecretProviderType.AWS,
    profile: undefined,
    vaultUrl: undefined,
    ...overrides,
  };
}

Usage:

typescript

// Arrange
const mapFile = createParsedMapFile({
  mappings: new Map([['API_KEY', '/prod/api-key']]),
});

Port Test Double Pattern

Build complete mock objects implementing domain interfaces:

typescript

function createMockSecretProvider(): ISecretProvider & {
  getSecrets: Mock;
} {
  return {
    getSecrets: vi.fn().mockResolvedValue(new Map()),
  };
}

function createMockLogger(): ILogger & {
  info: Mock;
  warn: Mock;
  error: Mock;
} {
  return {
    info: vi.fn(),
    warn: vi.fn(),
    error: vi.fn(),
  };
}

Summary

Double Type	Vitest API	Purpose
Mock	`vi.fn()`	Controllable replacement
Stub	`.mockResolvedValue()` / `.mockReturnValue()`	Predefined responses
Spy	`vi.spyOn()`	Observe real objects
Module mock	`vi.mock()`	Replace entire modules
Error sim	`.mockRejectedValue()`	Failure paths

When writing tests:

Create port test doubles with vi.fn() in beforeEach
Configure stubs with .mockResolvedValue() in Arrange
Always verify mock interactions in Assert
Use Mother pattern for complex test data
Prefer vi.fn() over vi.mock() (port-level > module-level)

Files

Just SKILL.md in .github/skills/typescript-test-doubles of macalbert/envilder.

Open the folder on GitHubat commit b6a0327

Compare with similar skills

Typescript Test Doubles next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Typescript Test Doubles compared with similar skills
Skill	Stars	Used in	Tokens	Auto-check	Licence	Repo updated
Typescript Test Doubles this skillmacalbert/envilder	138	—	~1.2k	Automated safety check: Pass	MIT	2 days ago
Test Writing WorkflowiOfficeAI/AionUi	33k	1 repos	~1.2k	Automated safety check: Pass	Apache-2.0	28 days ago
Creating A Packagec15t/c15t	1.9k	—	~913	Automated safety check: Pass	Apache-2.0	today
Svelte Testingspences10/sveltest	113	—	~579	Automated safety check: Pass	MIT	yesterday
Effect TStellahq/opensession	392	—	~3.7k	Automated safety check: Pass	MIT	today
Ha Frontend Testinghome-assistant/frontend	5.7k	—	~1.7k	Automated safety check: Pass	Apache-2.0	today

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Questions about Typescript Test Doubles

What does Typescript Test Doubles do?

Test doubles with Vitest (vi.fn, vi.mock, vi.spyOn) and Mother pattern. Typescript Test Doubles is an agent skill from macalbert/envilder.spyOn) and Mother pattern.

When should I use Typescript Test Doubles?

Typescript Test Doubles fits situations like: creating test data; mocking dependencies; setting up module mocks in TypeScript tests.

How do I install Typescript Test Doubles in Claude Code?

Run `npx skills add macalbert/envilder --skill typescript-test-doubles -a claude-code`. Or copy the skill folder (.github/skills/typescript-test-doubles in macalbert/envilder) into .claude/skills/typescript-test-doubles in your project. Claude Code loads it when a task matches its description.

How do I install Typescript Test Doubles in Codex?

Run `npx skills add macalbert/envilder --skill typescript-test-doubles -a codex`. Or copy the skill folder (.github/skills/typescript-test-doubles in macalbert/envilder) into .agents/skills/typescript-test-doubles in your project. Codex loads it when a task matches its description.

Can I use Typescript Test Doubles in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add macalbert/envilder --skill typescript-test-doubles -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/typescript-test-doubles, .gemini/skills/typescript-test-doubles, .github/skills/typescript-test-doubles and .opencode/skills/typescript-test-doubles in your project.

What does Typescript Test Doubles need to run?

Going by SKILL.md and its folder, Typescript Test Doubles needs credentials named API_KEY. Our summary lists: A credential in API_KEY.

Does Typescript Test Doubles access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Typescript Test Doubles safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Typescript Test Doubles use?

Typescript Test Doubles is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Typescript Test Doubles use?

About 1.2k tokens (SKILL.md is roughly 4.8k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Typescript Test Doubles?

Skills that share tags, products or a category with Typescript Test Doubles: Test Writing Workflow (iOfficeAI/AionUi, 33k stars), Creating A Package (c15t/c15t, 1.9k stars), Svelte Testing (spences10/sveltest, 113 stars) and Effect TS (tellahq/opensession, 392 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Typescript Test Doubles?

macalbert (a GitHub user) maintains it in macalbert/envilder, which has 138 GitHub stars. The repository holds 30 skills in this directory. The repository was last updated on October 5, 2026.

Source: macalbert/envilder on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.

Install Typescript Test Doubles

Facts

At a glance

What it does

When your agent uses it

Example prompts

Requirements

Workflow steps

What it can do on your machine

Tool permissions

Runs code

Network

Credentials

Context cost

Safety

SKILL.md

Test Doubles (TypeScript)

Test Doubles Types

1. Mock: vi.fn()

2. Stub: mockResolvedValue / mockReturnValue

3. Spy: vi.spyOn()

4. Module Mock: vi.mock()

5. Error Simulation: mockRejectedValue

Verification Patterns

Basic Verification

Not Called

Call Order

Argument Matchers

Mother Pattern

Port Test Double Pattern

Summary

Files

Compare with similar skills

Similar skills

More from macalbert/envilder

Related

Questions about Typescript Test Doubles