Overview

Want to quickly master Vitest through working examples? This by-example guide teaches 95% of Vitest through 85 annotated code examples organized by complexity level.

What Is By-Example Learning?

By-example learning is an example-first approach where you learn through annotated, runnable code rather than narrative explanations. Each example is self-contained, immediately executable with npx vitest run, and heavily commented to show:

• What each line does - Inline comments explain the purpose and mechanism
• Expected behaviors - Using // => notation to show test outcomes
• Intermediate states - Mock states, assertion results, and configuration effects made visible
• Key takeaways - 1-2 sentence summaries of core concepts

This approach is ideal for experienced developers (Jest users, testing library veterans, or software engineers) who are familiar with testing concepts and want to quickly understand Vitest's API, Vite-native features, and unique capabilities through working code.

Unlike narrative tutorials that build understanding through explanation and storytelling, by-example learning lets you see the code first, run it second, and understand it through direct interaction. You learn by doing, not by reading about doing.

Learning Path

%% Color Palette: Blue #0173B2, Orange #DE8F05, Teal #029E73, Purple #CC78BC, Brown #CA9161
graph TD
    A["Beginner<br/>Examples 1-30<br/>Core Fundamentals"] --> B["Intermediate<br/>Examples 31-58<br/>Production Patterns"]
    B --> C["Advanced<br/>Examples 59-85<br/>Expert Mastery"]
 
    style A fill:#0173B2,stroke:#000000,stroke-width:2px,color:#fff
    style B fill:#DE8F05,stroke:#000000,stroke-width:2px,color:#fff
    style C fill:#029E73,stroke:#000000,stroke-width:2px,color:#fff

Progress from fundamentals through production patterns to expert mastery. Each level builds on the previous, increasing in sophistication and introducing more Vitest-specific features.

Coverage Philosophy

This by-example guide provides 95% coverage of Vitest through practical, annotated examples. The 95% figure represents the depth and breadth of concepts covered, not a time estimate -- focus is on outcomes and understanding, not duration.

What's Covered

• Core fundamentals - test/it/describe, expect assertions, test lifecycle hooks
• Assertion matchers - Truthiness, numbers, strings, arrays, objects, exceptions, snapshots
• Test organization - skip, only, todo, each, concurrent, test context
• Mocking - vi.fn, vi.spyOn, vi.mock, module mocking, timer mocks, mock implementations
• Async testing - Promises, async/await, callback-based tests, concurrent execution
• DOM testing - happy-dom/jsdom environments, React component testing, hook testing
• Coverage - c8/istanbul providers, threshold configuration, reporting
• Advanced configuration - Workspaces, reporters, environments, pool configuration
• Type testing - expectTypeOf, assertType for compile-time verification
• Custom matchers - expect.extend for domain-specific assertions
• Performance - Benchmarking, test sharding, dependency optimization
• CI/CD integration - Reporter configuration, parallel execution, watch mode patterns

What's NOT Covered

This guide focuses on learning-oriented examples, not problem-solving recipes or production deployment. For additional topics:

• Deep framework integrations - Specific CI/CD platforms, cloud testing services
• Storybook integration - Storybook-specific test configurations
• Legacy migration - Detailed Jest-to-Vitest migration scripts

The 95% coverage goal maintains humility -- no tutorial can cover everything. This guide teaches the core concepts that unlock the remaining 5% through your own exploration and project work.

How to Use This Guide

1. Sequential or selective - Read examples in order for progressive learning, or jump to specific topics when switching from Jest
2. Run everything - Execute examples with npx vitest run to see results yourself. Experimentation solidifies understanding.
3. Modify and explore - Change assertions, add mocks, break tests intentionally. Learn through experimentation.
4. Use as reference - Bookmark examples for quick lookups when you forget syntax or patterns
5. Complement with narrative tutorials - By-example learning is code-first; pair with comprehensive tutorials for deeper explanations

Best workflow: Open your editor in one window, this guide in another, terminal in a third. Run each example as you read it. When you encounter something unfamiliar, run the example, modify it, see what changes.

Relationship to Other Tutorials

Understanding where by-example fits in the tutorial ecosystem helps you choose the right learning path:

By Example vs. Quick Start: By Example provides 95% coverage through examples vs. Quick Start's 5-30% through your first test. By Example is code-first reference; Quick Start is hands-on introduction.

By Example vs. Beginner Tutorial: By Example is code-first for experienced developers; Beginner Tutorial is narrative-first for complete testing beginners. By Example shows patterns; Beginner Tutorial explains concepts.

By Example vs. Cookbook: By Example is learning-oriented (understand concepts); Cookbook is problem-solving oriented (fix specific issues). By Example teaches patterns; Cookbook provides solutions.

Prerequisites

Required:

• Experience with JavaScript/TypeScript development
• Ability to run Node.js commands and npm/npx
• Basic understanding of testing concepts (assertions, test structure)

Recommended (helpful but not required):

• Familiarity with Vite build tool
• Experience with another testing framework (Jest, Mocha, Jasmine)
• Understanding of ES modules vs CommonJS

No prior Vitest experience required -- This guide assumes you're new to Vitest but experienced with JavaScript/TypeScript development in general. You should be comfortable reading TypeScript code, understanding basic testing concepts (assertions, mocks, test lifecycle), and learning through hands-on experimentation.

Structure of Each Example

Every example follows a mandatory five-part format:

### Example N: Concept Name
 
**Part 1: Brief Explanation** (2-3 sentences)
Explains what the concept is, why it matters in testing, and when to use it.
 
**Part 2: Mermaid Diagram** (when appropriate)
Visual representation of concept relationships - test flow, mock hierarchies, or configuration composition. Not every example needs a diagram; they're used strategically to enhance understanding.
 
**Part 3: Heavily Annotated Code**
 
```typescript
import { describe, it, expect } from "vitest";
// => Vitest test framework imports
// => describe: group tests, it: define test, expect: assertions
 
describe("example suite", () => {
  // => describe() groups related tests
  // => String label appears in test output
 
  it("should demonstrate concept", () => {
    // => it() defines a single test case
    // => String describes expected behavior
 
    const result = 2 + 2;
    // => Performs calculation
    // => result is 4 (type: number)
 
    expect(result).toBe(4);
    // => Asserts result equals 4 (strict equality)
    // => Test passes when values match
  });
});
```
 
**Part 4: Key Takeaway** (1-2 sentences)
Distills the core insight: the most important pattern, when to apply it in production, or common pitfalls to avoid.
 
**Part 5: Why It Matters** (2-3 sentences, 50-100 words)
Connects the concept to production relevance - why professionals care, how it compares to alternatives, and consequences for quality/performance/maintainability.

Each example follows this structure consistently, maintaining annotation density of 1.0-2.25 comment lines per code line. The brief explanation provides context, the code is heavily annotated with inline comments and // => output notation, the key takeaway distills the concept, and why it matters shows production relevance.

Learning Strategies

For Jest Users

You're familiar with Jest's API and patterns. Vitest provides near-identical API with Vite-native performance:

• Same API: describe, it, expect, vi (replaces jest) -- minimal rewriting
• Native ESM: No transform configuration for ES modules
• Vite integration: Shared config with your build tool, HMR-powered watch mode

Focus on Examples 1-5 (basic setup and API differences) and Examples 31-40 (module mocking with vi.mock) to identify key differences from Jest.

For Mocha/Chai Users

You understand BDD-style testing and assertion libraries. Vitest combines test runner and assertions:

• Built-in assertions: No separate assertion library needed
• BDD syntax: describe/it work exactly as expected
• Integrated mocking: vi.fn, vi.mock built into the framework

Focus on Examples 6-15 (assertion matchers) and Examples 20-25 (mocking basics) to see how Vitest unifies the testing experience.

For Frontend Developers

You build with Vite and want testing that matches your build tool:

• Shared configuration: vitest.config.ts extends vite.config.ts
• Same transforms: TypeScript, JSX, CSS modules work identically in tests
• Component testing: First-class happy-dom/jsdom support

Focus on Examples 44-48 (DOM and component testing) and Examples 50-53 (coverage configuration) to integrate testing into your Vite workflow.

For Backend/Node.js Developers

You test APIs, services, and utilities. Vitest provides fast, modern testing:

• TypeScript native: No separate ts-jest configuration
• Fast execution: Vite's transformation pipeline is significantly faster
• Module mocking: vi.mock handles ESM and CJS modules cleanly

Focus on Examples 16-19 (async testing) and Examples 31-38 (advanced mocking) to apply Vitest to backend code.

Code-First Philosophy

This tutorial prioritizes working code over theoretical discussion:

• No lengthy prose: Concepts are demonstrated, not explained at length
• Runnable examples: Every example runs with npx vitest run
• Learn by doing: Understanding comes from running and modifying code
• Pattern recognition: See the same patterns in different contexts across 85 examples

If you prefer narrative explanations, complement this guide with comprehensive tutorials. By-example learning works best when you learn through experimentation.

Ready to Start?

Jump into the beginner examples to start learning Vitest through code:

• Beginner Examples (1-30) - Core fundamentals, assertions, mocking basics, async testing
• Intermediate Examples (31-58) - Module mocking, DOM testing, coverage, workspaces, custom matchers
• Advanced Examples (59-85) - Benchmarking, browser mode, sharding, CI/CD, advanced patterns

Each example is self-contained and runnable. Start with Example 1, or jump to topics that interest you most.