How do you write Jest tests for LWC components?

LWC supports unit testing via Jest running locally in Node.js — no Salesforce org needed.

Setup:

bash npm install sf force lightning lwc test setup # one-time setup

Generates jest.config.js and force-app/main/default/lwc/.../mycomponent.test.js files.

Anatomy of a Jest test:

`javascript import { createElement } from 'lwc'; import MyComponent from 'c/myComponent';

describe('c-my-component', () => { afterEach(() => { // Reset DOM between tests while (document.body.firstChild) { document.body.removeChild(document.body.firstChild); } });

it('renders the title', () => { const element = createElement('c-my-component', { is: MyComponent }); element.title = 'Hello'; document.body.appendChild(element);

const h1 = element.shadowRoot.querySelector('h1'); expect(h1.textContent).toBe('Hello'); });

it('dispatches a userselected event on click', () => { const element = createElement('c-my-component', { is: MyComponent }); document.body.appendChild(element);

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

const button = element.shadowRoot.querySelector('button'); button.click();

expect(handler).toHaveBeenCalled(); }); }); `

Mocking Apex methods:

`javascript import getAccount from '@salesforce/apex/AccountController.getAccount';

jest.mock('@salesforce/apex/AccountController.getAccount', () => ({ default: jest.fn() }), { virtual: true });

it('handles wire data', async () => { getAccount.mockResolvedValue({ Id: '001x', Name: 'Acme' }); const element = createElement('c-my-component', { is: MyComponent }); document.body.appendChild(element); await Promise.resolve(); // wait for wire to resolve

const span = element.shadowRoot.querySelector('span'); expect(span.textContent).toBe('Acme'); }); `

Mocking Lightning Data Service:

Use @salesforce/sfdx-lwc-jest mocks for getRecord, getRecordCreateDefaults, etc.

Mocking Lightning Message Service:

createElement + subscribe/publish mocks.

Patterns:

• Test the public API only — @api properties and dispatched events. Don't test private state.
• Async resolution — wires resolve on next microtask; use await Promise.resolve() or await flushPromises() between actions and assertions.
• No `connectedCallback` race — the component renders synchronously after appendChild.
• Coverage — Jest reports coverage; pair with PMD on Apex for full project metrics.
• CI integration — Jest runs in PR validation pipelines, headless.

Anti-patterns:

• Testing implementation details (private state, internal method names).
• Tests that depend on real Salesforce — use mocks.
• Snapshot tests of HTML output — fragile, low value.
• Skipping accessibility checks — use @testing-library/jest-dom or axe for a11y.

LWC + Jest brings front-end testing maturity to Salesforce. Most LWC-heavy orgs aim for 70-80% LWC test coverage alongside 75% Apex coverage.