Imported from grzmol/vibe-force (skills/sf-lwc-jest-testing/SKILL.md). Install upstream with npx skills add grzmol/vibe-force --skill sf-lwc-jest-testing. Copyright stays with the author.

LWC Jest Testing

When to use

Jest runs entirely offline: no org, no authentication, no network. It is the cheapest gate in the pipeline and runs in wave 2 as part of vf-check local.

Quick reference

Core patterns

1. Project setup

{
  "devDependencies": {
    "@salesforce/sfdx-lwc-jest": "^7.0.0"
  },
  "scripts": {
    "test": "npm run test:unit",
    "test:unit": "sfdx-lwc-jest",
    "test:unit:watch": "sfdx-lwc-jest --watch",
    "test:unit:debug": "sfdx-lwc-jest --debug",
    "test:unit:coverage": "sfdx-lwc-jest --coverage"
  }
}

// jest.config.js at the root of the Salesforce DX project
const { jestConfig } = require('@salesforce/sfdx-lwc-jest/config');

module.exports = {
    ...jestConfig,
    moduleNameMapper: {
        '^@salesforce/apex$': '<rootDir>/force-app/test/jest-mocks/apex',
        '^lightning/navigation$': '<rootDir>/force-app/test/jest-mocks/lightning/navigation',
        '^lightning/platformShowToastEvent$':
            '<rootDir>/force-app/test/jest-mocks/lightning/platformShowToastEvent',
        '^lightning/messageService$': '<rootDir>/force-app/test/jest-mocks/lightning/messageService',
        '^lightning/uiRecordApi$': '<rootDir>/force-app/test/jest-mocks/lightning/uiRecordApi'
    },
    collectCoverageFrom: ['force-app/main/default/lwc/**/*.js', '!**/__tests__/**'],
    coverageThreshold: { global: { lines: 80, statements: 80 } },
    testTimeout: 10000
};

sfdx-lwc-jest configures Jest for the DX workspace out of the box; jest.config.js is only needed to override. Stubs for every lightning namespace base component ship with the package and are used automatically. The coverageThreshold mirrors gates.jestCoverageMin (80) from .vibeforce/config.json; vf-check jest enforces the same number independently.

2. Test anatomy

// force-app/main/default/lwc/hello/__tests__/hello.test.js
import { createElement } from 'lwc';
import Hello from 'c/hello';

describe('c-hello', () => {
    afterEach(() => {
        // The jsdom instance is shared across test cases in a single file, so reset the DOM
        while (document.body.firstChild) {
            document.body.removeChild(document.body.firstChild);
        }
        jest.clearAllMocks();
    });

    it('displays the greeting', () => {
        const element = createElement('c-hello', { is: Hello });
        document.body.appendChild(element);

        const div = element.shadowRoot.querySelector('div');
        expect(div.textContent).toBe('Hello, World!');
    });
});

Rules:

• The tag name passed to createElement must be the kebab-case name with the c- namespace.
• Nothing renders until document.body.appendChild(element).
• Never share an element between it blocks.

3. Asynchronous rerenders

// helper used across the suite
async function flushPromises() {
    return Promise.resolve();
}

it('updates the label when the property changes', async () => {
    const element = createElement('c-counter', { is: Counter });
    document.body.appendChild(element);

    element.count = 5;                     // property set after insertion

    await flushPromises();                 // the rerender is enqueued as a microtask

    expect(element.shadowRoot.querySelector('.label').textContent).toBe('5');
});

One await Promise.resolve() flushes one microtask turn. Chained promises (an imperative Apex call that then triggers a rerender) need one flush per turn, or await Promise.resolve().then(() => Promise.resolve()). Never use setTimeout to "wait for" a rerender.

4. Mocking Apex

import { createElement } from 'lwc';
import CaseList from 'c/caseList';
import getCases from '@salesforce/apex/CaseController.getCases';

// The LWC Jest transformer resolves @salesforce/apex/* modules; declare the mock as virtual.
jest.mock(
    '@salesforce/apex/CaseController.getCases',
    () => ({ default: jest.fn() }),
    { virtual: true }
);

const MOCK_CASES = [{ Id: '500xx0000000001', Subject: 'Broken widget' }];

describe('c-case-list imperative Apex', () => {
    afterEach(() => {
        while (document.body.firstChild) document.body.removeChild(document.body.firstChild);
        jest.clearAllMocks();
    });

    it('renders rows on success', async () => {
        getCases.mockResolvedValue(MOCK_CASES);

        const element = createElement('c-case-list', { is: CaseList });
        document.body.appendChild(element);
        element.shadowRoot.querySelector('lightning-button').click();

        await Promise.resolve();
        await Promise.resolve();

        expect(getCases).toHaveBeenCalledWith({ ownerId: undefined, maxRows: 25 });
        expect(element.shadowRoot.querySelectorAll('tr')).toHaveLength(1);
    });

    it('renders an error panel on failure', async () => {
        getCases.mockRejectedValue({ body: { message: 'Insufficient access' }, ok: false, status: 400 });

        const element = createElement('c-case-list', { is: CaseList });
        document.body.appendChild(element);
        element.shadowRoot.querySelector('lightning-button').click();

        await Promise.resolve();
        await Promise.resolve();

        expect(element.shadowRoot.querySelector('c-error-panel')).not.toBeNull();
    });
});

Assert both the success and the failure path for every server call - the error branch is where production bugs live.

5. Driving wire adapters

@salesforce/sfdx-lwc-jest re-exports the @salesforce/wire-service-jest-util APIs, so no extra dependency is required.

import { createElement } from 'lwc';
import AccountCard from 'c/accountCard';
import { getRecord } from 'lightning/uiRecordApi';
import getOpenCases from '@salesforce/apex/AccountCaseController.getOpenCases';

const mockGetRecord = require('./data/getRecord.json');

jest.mock(
    '@salesforce/apex/AccountCaseController.getOpenCases',
    () => {
        const { createApexTestWireAdapter } = require('@salesforce/sfdx-lwc-jest');
        return { default: createApexTestWireAdapter(jest.fn()) };
    },
    { virtual: true }
);

it('renders the account name from the wire', async () => {
    const element = createElement('c-account-card', { is: AccountCard });
    element.recordId = '001xx000003DGg0AAG';
    document.body.appendChild(element);

    getRecord.emit(mockGetRecord);         // lightning/uiRecordApi is stubbed as a test adapter
    getOpenCases.emit([{ Id: '500xx1', Subject: 'Late shipment' }]);

    await Promise.resolve();

    expect(element.shadowRoot.querySelector('.name').textContent).toBe('Acme');
    expect(getRecord.getLastConfig().recordId).toBe('001xx000003DGg0AAG');
});

Error paths: getRecord.error() emits the default not-found response; getRecord.emitError({ body: { message: 'Boom' }, status: 500, statusText: 'Server Error' }) emits a specific one.

6. Asserting dispatched events

it('dispatches rowselect with the record id', async () => {
    const element = createElement('c-selector', { is: Selector });
    document.body.appendChild(element);

    const handler = jest.fn();
    element.addEventListener('rowselect', handler);

    element.shadowRoot.querySelector('[data-id="003xx1"]').click();
    await Promise.resolve();

    expect(handler).toHaveBeenCalledTimes(1);
    expect(handler.mock.calls[0][0].detail).toEqual({ recordId: '003xx1' });
    expect(handler.mock.calls[0][0].bubbles).toBe(false);
});

For toasts, listen for ShowToastEventName imported from lightning/platformShowToastEvent.

7. What to assert, and what not to

A test that would still pass after the component is broken is worse than no test. Snapshot tests are allowed only for stable presentational markup; a snapshot that is regenerated with -u every sprint is noise and must be deleted.

8. Timers and intervals

it('ticks every second', () => {
    jest.useFakeTimers();
    const element = createElement('c-clock', { is: Clock });
    document.body.appendChild(element);

    jest.advanceTimersByTime(3000);

    expect(element.shadowRoot.querySelector('.tick').textContent).toBe('3');
    document.body.removeChild(element);
    jest.advanceTimersByTime(3000);          // proves disconnectedCallback cleared the interval
    expect(element.shadowRoot.querySelector('.tick').textContent).toBe('3');
    jest.useRealTimers();
});

Anti-patterns

Verification

# All LWC tests
node "${CLAUDE_PLUGIN_ROOT}/scripts/checks/vf-check.mjs" jest

# Only tests affected by the current branch
node "${CLAUDE_PLUGIN_ROOT}/scripts/checks/vf-check.mjs" jest --changed

# Full local gate (format + lint + analyzer + jest)
node "${CLAUDE_PLUGIN_ROOT}/scripts/checks/vf-check.mjs" local --changed

# Direct invocations while iterating
npm run test:unit -- force-app/main/default/lwc/accountCard/__tests__/accountCard.test.js
npm run test:unit:watch
npm run test:unit:coverage
npm run test:unit:debug            # then open chrome://inspect
sfdx-lwc-jest -- --runInBand       # everything after -- goes straight to Jest

vf-check jest fails the wave-2 gate when any test fails, when coverage is below gates.jestCoverageMin (80), or when gates.requireJestForLwc is true and a bundle has no __tests__ directory. No org is contacted; exit code 1 means a real gate failure, 2 means the toolchain is missing (run npm install in the consumer project).

References

• references/jest-setup.md - full jest.config.js, npm scripts, __mocks__ layout, CI invocation, coverage configuration.
• references/mocking-cookbook.md - Apex, LDS, GraphQL, navigation, LMS, toast, labels, base components, fetch.
• references/test-recipes.md - complete test files for the component shapes this harness builds.
• references/troubleshooting-jest.md - failure-to-cause table and debugging workflow.
• Official: sfdx-lwc-jest README, wire-service-jest-util README, Test Lightning Web Components, Jest 30 documentation.