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of 49How do you install and set up Jest in a project?
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Jest can be installed via npm or yarn:
npm install --save-dev jest
# or
yarn add --dev jest
Basic setup in package.json:
{
"scripts": {
"test": "jest",
"test:watch": "jest --watch",
"test:coverage": "jest --coverage"
}
}
For projects without Babel, you might need additional configuration for ES6+ syntax.
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What is the difference between `test()` and `it()` in Jest?
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There is no functional difference between test() and it() - they are aliases of each other. Both are used to define individual test cases:
test('should add two numbers', () => {
expect(add(2, 3)).toBe(5);
});
it('should add two numbers', () => {
expect(add(2, 3)).toBe(5);
});
The choice between them is often stylistic. it() reads more naturally in BDD (Behavior Driven Development) style, while test() is more explicit about its purpose.
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What is `describe()` and when should you use it?
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describe() is used to group related tests together. It creates a test suite and helps organize tests logically:
describe('Calculator', () => {
test('should add two numbers', () => {
expect(add(2, 3)).toBe(5);
});
test('should subtract two numbers', () => {
expect(subtract(5, 3)).toBe(2);
});
});
Benefits of using describe():
- Better test organization and readability
- Scoped setup and teardown hooks
- Grouped test output in reports
- Easier to run specific test suites
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How do you structure a typical Jest test file?
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A well-structured Jest test file typically follows this pattern:
// Import dependencies
import { functionToTest } from '../src/utils';
// Describe the module/component being tested
describe('functionToTest', () => {
// Group related tests
describe('when given valid input', () => {
test('should return expected result', () => {
// Arrange
const input = 'test';
// Act
const result = functionToTest(input);
// Assert
expect(result).toBe('expected');
});
});
describe('when given invalid input', () => {
test('should throw an error', () => {
expect(() => functionToTest(null)).toThrow();
});
});
});
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What are the different ways to run Jest tests?
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Jest can be run in several ways:
- Basic test run:
npm testorjest - Watch mode:
jest --watch(watches for file changes) - Watch all:
jest --watchAll(watches all files) - Coverage:
jest --coverage(generates coverage report) - Specific file:
jest mytest.test.js - Pattern matching:
jest --testNamePattern="should add" - Verbose output:
jest --verbose
Common package.json scripts:
{
"scripts": {
"test": "jest",
"test:watch": "jest --watch",
"test:coverage": "jest --coverage",
"test:debug": "jest --verbose"
}
}
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What are the most commonly used Jest matchers?
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Jest provides many built-in matchers for different assertion types:
Equality matchers:
toBe(): Exact equality (Object.is)toEqual(): Deep equalitytoStrictEqual(): Strict deep equality
Truthiness matchers:
toBeTruthy(): Truthy valuestoBeFalsy(): Falsy valuestoBeNull(): Specifically nulltoBeUndefined(): Specifically undefined
Number matchers:
toBeGreaterThan(): Greater thantoBeCloseTo(): Floating point numbers
String matchers:
toMatch(): Regular expressionstoContain(): Substring matching
Array/Object matchers:
toContain(): Array contains itemtoHaveProperty(): Object has property
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What's the difference between `toBe()` and `toEqual()`?
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toBe() uses Object.is() for exact equality (reference equality for objects), while toEqual() performs deep equality checking:
test('toBe vs toEqual', () => {
const obj1 = { name: 'John' };
const obj2 = { name: 'John' };
const obj3 = obj1;
// toBe checks reference equality
expect(obj1).toBe(obj3); // ✓ Same reference
expect(obj1).toBe(obj2); // ✗ Different references
// toEqual checks deep equality
expect(obj1).toEqual(obj2); // ✓ Same content
expect(obj1).toEqual(obj3); // ✓ Same content
// For primitives, they work similarly
expect(5).toBe(5); // ✓
expect(5).toEqual(5); // ✓
});
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How do you test for exceptions in Jest?
How do you test arrays and objects with Jest?
What is mocking in Jest and why is it important?
How do you create and use mock functions in Jest?
What's the difference between `jest.mock()` and `jest.spyOn()`?
How do you mock modules and external dependencies?
How do you test asynchronous code in Jest?
How do you test setTimeout and setInterval functions?
What are Jest lifecycle hooks and when should you use them?
How do you share setup code across multiple test files?
How do you configure Jest for different environments?
What is the purpose of `setupFilesAfterEnv` in Jest configuration?
How do you test React components with Jest and React Testing Library?
What is snapshot testing and when should you use it?
How do you test modules that use ES6 imports/exports?
How do you handle code coverage in Jest?
How do you mock global objects like `window` or `localStorage`?
How do you debug Jest tests?
How do you test React components with hooks using Jest?
How do you mock React hooks in Jest?
How do you test async/await functions with error handling?
What is Jest's module factory pattern?
How do you test React components with context providers?
How do you mock environment variables in Jest?
How do you test code that uses Web APIs like localStorage?
How do you create custom matchers in Jest?
What are some Jest testing best practices?
How do you handle flaky tests in Jest?
What's the difference between integration and unit tests in Jest?
What are Jest transformers and when do you need them?
How do you test components with lazy loading and Suspense?
How do you implement and test custom Jest matchers for domain-specific assertions?
How do you test WebSocket connections with Jest?
How do you test infinite scroll or pagination components?
How do you test React components with drag and drop functionality?
How do you test components that use requestAnimationFrame?
How do you test components with complex state management (Redux/Zustand)?
How do you test components with dynamic imports and code splitting?
How do you test components with performance optimization (React.memo, useMemo)?
How do you test accessibility features and ARIA attributes with Jest?
How do you test micro-frontend components and cross-app communication?
How do you implement comprehensive integration testing with Jest for full user workflows?
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