jest-unit-testing

A set of rules for writing Jest unit tests, which check small parts of a JavaScript or TypeScript program in isolation. It specifies where tests go, what cases to cover, and how to run them.

In plain words
What is it for?
Use it to test business logic, utility functions, service classes, and components with focused tests covering valid inputs, invalid inputs, and edge cases.
Why use it?
It gives developers a consistent way to test important logic and catch invalid or unusual inputs without writing broad, unfocused test files.

Cursor rule for Cursor

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add rules/consensys/ask-o11y-plugin/jest-unit-testing
Clone the repo
git clone --depth 1 https://github.com/Consensys/ask-o11y-plugin

Made for: Cursor.

Per session 0 Nothing until a file matches its globs; then the whole rule loads.
When invoked 249 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00000 $0.00249
Opus 5 $0.00000 $0.00125
Sonnet 5 $0.00000 $0.00050
Haiku 4.5 $0.00000 $0.00025

Measured 3d ago against content hash 71fc56713a91, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

jest-unit-testing scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 3d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.cursor/rules/jest-unit-testing.mdc · 34 lines

What it actually says

Jest Unit Testing

Reference: CLAUDE.md is the authoritative source.

Test Locations

  • Unit tests in src/**/*.test.{ts,tsx} alongside the code they test
  • Use data-testid from src/components/testIds.ts for component references

Focus

Test critical functionality: business logic, utility functions, service classes. Limit to 3–5 focused tests per file.

Best Practices

  1. Mock dependencies before imports with jest.mock()
  2. Test three scenarios: valid inputs, invalid inputs, edge cases (null, undefined, unexpected types)
  3. Descriptive names — test name should describe expected behavior
  4. Group related tests in describe blocks
  5. Clear mocks in beforeEach with jest.clearAllMocks()

Run Tests

nvm use 22 && npm run test:ci              # CI mode
nvm use 22 && npm test -- path/to/test.ts  # Single file
Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 3d ago First seen · 34 lines · 0 tokens per session scan A 71fc56713a91

Subscribe to this mod's changes

jest-unit-testing is a cursor rule published in the GitHub repository Consensys/ask-o11y-plugin (40 stars, last pushed 8d ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 249 tokens. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.