How to set up nyc with Mocha for coverage?

Install nyc as a dev dependency, then modify your package.json scripts to run mocha under nyc, e.g., 'nyc mocha'. You can add reporters like lcov or text-summary via CLI or config files for customized outputs.

nyc vs c8: which is better for JavaScript coverage?

nyc is the standard CLI for Istanbul with extensive features and ecosystem support, ideal for complex configurations. c8 uses V8's built-in coverage and might be faster but less configurable, so choose based on project needs.

How to exclude node_modules from nyc coverage?

By default, nyc excludes node_modules, but you can override this with 'excludeNodeModules: false' in your config. Use include and exclude arrays with glob patterns to fine-tune file selection, as explained in the README.

Why is my TypeScript coverage not showing correctly in nyc?

Ensure you're using the @istanbuljs/nyc-config-typescript preset and that source maps are properly configured. If pre-instrumenting code, set '--exclude-after-remap=false' to avoid exclusion issues.

How to combine coverage from multiple test runs in nyc?

Use the '--no-clean' flag on subsequent runs to preserve raw coverage data, then run 'nyc report' to merge and generate a final report. The README provides an example with separate unit and integration test scripts.

Best way to configure nyc for a Babel project?

Start with the @istanbuljs/nyc-config-babel preset, then add custom options like 'all: true' or specific reporters in your nyc config file. This ensures proper instrumentation and source map handling for transpiled code.

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nyc

ISCJavaScriptnyc-v18.0.0

A command-line interface for Istanbul that instruments JavaScript code to collect test coverage metrics.

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5.8k stars360 forks0 contributors

What is nyc?

nyc is the command-line interface for Istanbul, a JavaScript code coverage tool. It instruments JavaScript code to track which lines, branches, and functions are executed during tests, generating coverage reports to help developers assess test effectiveness. It supports modern JavaScript features, source maps for transpiled code, and integrates with popular test runners.

Target Audience

JavaScript developers and teams using test runners like Mocha, AVA, or Tape who need to measure and improve their test coverage, especially in projects using Babel or TypeScript.

Value Proposition

Developers choose nyc for its deep integration with the Istanbul ecosystem, extensive configuration options, support for complex scenarios like subprocesses and source maps, and its ability to work with almost any JavaScript test runner without requiring major workflow changes.

Overview

the Istanbul command line interface

Use Cases

Best For

Measuring test coverage in Node.js applications
Tracking coverage for Babel-transpiled JavaScript projects
Generating coverage reports for TypeScript codebases
Enforcing coverage thresholds in CI/CD pipelines
Combining coverage from multiple test suites or runners
Instrumenting code that spawns child processes

Not Ideal For

Projects using Jest or Tap as their primary test runner, since these include built-in coverage tools
Teams seeking a zero-configuration, out-of-the-box coverage solution with minimal setup
Applications requiring real-time coverage feedback during development, as nyc is designed for post-run reporting
Small-scale scripts or projects where the overhead of instrumentation and reporting is disproportionate to benefits

Pros & Cons

Pros

Subprocess Coverage

nyc uniquely collects coverage from applications that spawn subprocesses, essential for complex Node.js workflows, as highlighted in the README's key features.

Source Map Support

Accurately handles coverage for Babel and TypeScript projects via source maps, ensuring reports reflect the original source code, as emphasized in the README's description.

Flexible Configuration

Offers multiple configuration methods including CLI, package.json, and various config files, allowing fine-grained control, with examples provided for JSON, YAML, and JS formats.

Coverage Threshold Enforcement

Enables setting custom coverage thresholds to fail builds if not met, integrating seamlessly with CI/CD pipelines, as detailed in the coverage thresholds section.

Cons

Complex Setup for Transpiled Code

Requires additional presets and careful configuration for Babel or TypeScript, which can be error-prone and time-consuming, as the README advises using specific config packages.

Cache Management Issues

The caching feature, while improving performance, can lead to stale coverage data if not properly invalidated, and the README lacks clear guidance on cache invalidation strategies.

Verbose and Scattered Documentation

Advanced features are documented across multiple pages, making it challenging for users to find comprehensive information without deep diving, as seen with links to external tutorials.

Frequently Asked Questions

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