Stryker is a mutation testing framework for JavaScript and TypeScript. It helps developers ensure the quality of their tests by introducing small changes (mutations) to the source code and checking if the tests still pass. This helps identify weaknesses in the test suite and improve the overall test coverage.

Pros

• Comprehensive Mutation Testing: Stryker supports a wide range of mutation operators, allowing for thorough testing of the codebase.
• Language Support: Stryker supports both JavaScript and TypeScript, making it applicable to a wide range of projects.
• Customizable Configuration: Stryker provides a flexible configuration system, allowing users to tailor the mutation testing process to their specific needs.
• Integrations: Stryker integrates with popular testing frameworks and CI/CD tools, making it easy to incorporate into existing development workflows.

Cons

• Performance Impact: Mutation testing can be computationally intensive, especially for large codebases, which may slow down the development process.
• Learning Curve: Configuring and using Stryker effectively may require some initial effort, as it introduces a new concept (mutation testing) to the development workflow.
• Potential False Positives: Mutation testing can sometimes identify "false positive" issues, where the tests pass but the mutated code is still incorrect, requiring manual investigation.
• Ecosystem Maturity: Compared to other testing tools, the Stryker ecosystem may be less mature, with fewer resources and community support available.

Code Examples

// Example 1: Configuring Stryker
const config = {
  mutate: ['src/**/*.js'],
  testRunner: 'jest',
  reporters: ['html', 'clear-text', 'progress'],
  coverageAnalysis: 'perTest',
};

module.exports = config;

This code snippet shows an example Stryker configuration file, where the user specifies the files to mutate, the test runner to use, the reporters to generate output, and the coverage analysis strategy.

// Example 2: Defining a Mutation Operator
class ConstantFolding extends Mutator {
  static mutatorName = 'ConstantFolding';

  mutate(node) {
    if (node.type === 'BinaryExpression') {
      return this.mutateBinaryExpression(node);
    }
    return null;
  }

  mutateBinaryExpression(node) {
    const left = this.evaluateNode(node.left);
    const right = this.evaluateNode(node.right);
    const operator = node.operator;

    const result = this.calculateResult(left, right, operator);
    return t.numericLiteral(result);
  }

  // ...
}

This code example demonstrates how to define a custom mutation operator in Stryker. The ConstantFolding operator mutates binary expressions by evaluating the left and right operands and replacing the expression with a numeric literal.

// Example 3: Using Stryker in a TypeScript Project
import { StrykerOptions } from '@stryker-mutator/api/core';

const config: StrykerOptions = {
  mutate: ['src/**/*.ts'],
  testRunner: 'jest',
  reporters: ['html', 'clear-text', 'progress'],
  coverageAnalysis: 'perTest',
  tsconfigFile: 'tsconfig.json',
};

module.exports = config;

This TypeScript example shows how to configure Stryker for a TypeScript project, including specifying the tsconfigFile option to ensure proper handling of TypeScript-specific features.

Getting Started

To get started with Stryker, follow these steps:

1. Install Stryker and its dependencies:

   npm install --save-dev @stryker-mutator/core @stryker-mutator/javascript-mutator @stryker-mutator/jest-runner
   

2. Create a Stryker configuration file (e.g., stryker.conf.js) in the root of your project:

   module.exports = {
     mutate: ['src/**/*.js'],
     test