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SWC-coverage-instrument

swc-coverage-instrument is a set of packages to support istanbuljs compatible coverage instrumentation in SWC's transform passes. Instrumentation transform can be performed either via SWC's wasm-based plugin, or using custom passes in rust side transform chains.

What does compatible exactly means?

This instrumentation will generate a data struct mimics istanbuljs's FileCoverage object conforms fixture test suite from istanbuljs itself.

However, this doesn't mean instrumentation supports exact same interfaces surrounding coverage object as well as supporting exact same options. There are some fundamental differences between runtime, and ast visitor architecture between different compilers does not allow identical behavior. This package will try best attempt as possible.

NOTE: Package can have breaking changes without major semver bump

Given SWC's plugin interface itself is under experimental stage does not gaurantee semver-based major bump yet, this package also does not gaurantee semver compliant breaking changes yet. Please refer changelogs if you're encountering unexpected breaking behavior across versions.

Usage

Using SWC's wasm-based experimental plugin

First, install package via npm:

npm install --save-dev swc-plugin-coverage-instrument

Then add plugin into swc's configuration:

const pluginOptions: InstrumentationOptions = {...}

jsc: {
  ...
  experimental: {
    plugins: [
      ["swc-plugin-coverage-instrument", pluginOptions]
    ]
  }
}

InstrumentationOptions is a subset of istanbul's instrumentation options. Refer istanbul's option for the same configuration flags. However there are few exceptions or differences, referencing InstrumentOptions will list all possible options.

interface InstrumentationOptions {
  coverageVariable?: String,
  compact?: bool,
  reportLogic?: bool,
  ignoreClassMethods?: Array<String>,
  inputSourceMap?: object,
  instrumentLog: {
    // Currently there aren't logs other than spans.
    // Enabling >= info can display span traces.
    level: 'trace' | 'warn' | 'error' | 'info'
    // Emits spans along with any logs
    // Only effective if level sets higher than info.
    enableTrace: bool
  },
  unstableExclude?: Array<String>
}

Using custom transform pass in rust

There is a single interface exposed to create a visitor for the transform, which you can pass into before_custom_pass.

let visitor = swc_coverage_instrument::create_coverage_instrumentation_visitor(
    source_map: std::sync::Arc<SourceMapper>,
    comments: C,
    instrument_options: InstrumentOptions,
    filename: String,
);

let fold = as_folder(visitor);

Building / Testing

This package runs istanbuljs' fixture tests against SWC with its wasm plugin & custom transform both. spec contains set of the fixtures & unit test to run it, as well as supplimental packages to interop between instrumentation visitor to node.js runtime. swc-coverage-instrument-wasm exposes FileCoverageInterop allows to consume FileCoverage struct inside of js, and swc-coverage-custom-transform is an example implementation to run before_custom_pass with swc-coverage-instrument visitor.

Few npm scripts are supported for wrapping those setups.