@swc/jest

@swc/jest is Jest’s official-style integration for SWC, a Rust-based JavaScript/TypeScript compiler. Registered as a transform in jest.config.js, it replaces slower Babel- or ts-jest-based transforms with SWC’s native compilation, while still reading project .swcrc configuration, computing Jest-compatible cache keys, and supporting Jest’s code-coverage instrumentation hooks.

What You Get

• A drop-in Jest transform registered via the transform config key, replacing babel-jest or ts-jest
• Automatic .swcrc config loading with JSONC parsing, or inline transform options passed from jest.config.js
• Correct ESM vs CommonJS module output based on Jest’s supportsStaticESM flag per test file
• Coverage instrumentation support via automatic injection of the swc-plugin-coverage-instrument SWC plugin when Jest runs with —coverage

Common Use Cases

• Speeding up a large Jest test suite’s TypeScript/JSX compilation by swapping ts-jest or babel-jest for SWC
• Running Jest against ESM-authored source files with correct static-ESM output per Jest’s per-file module detection
• Collecting accurate code coverage from SWC-compiled tests via the custom coverage instrumentation option
• Sharing a single .swcrc configuration between a project’s build pipeline and its Jest test transform

Under The Hood

Architecture: index.ts exports createTransformer(), whose returned object implements Jest’s Transformer interface: process()/processAsync() call @swc/core’s transformSync/transform respectively, always overriding the module.type option per-file based on Jest’s supportsStaticESM flag, while getCacheKey() combines the SWC version, package version, and computed SWC options into a Jest-compatible cache key via @jest/create-cache-key-function; a separate buildSwcTransformOpts() helper merges explicit options with .swcrc (parsed via jsonc-parser to tolerate comments), defaults jsc.target from the running Node version, and forces jsc.transform.hidden.jest plus inline sourceMaps. Tech Stack: TypeScript compiled with tsc as part of a pnpm/Turborepo-style monorepo (swc-project/pkgs) alongside swc-loader and @swc/cli, with @swc/core and @jest/transform as its core dependency/peer surface and jsonc-parser for tolerant .swcrc parsing. Code Quality: The transformer is a single well-scoped 215-line index.ts with small pure helper functions (nodeTarget, insertInstrumentationOptions, set) and no classes; the monorepo runs eslint across packages, though this specific package’s own dedicated test suite is thin relative to its role as a widely-depended-on transform, relying more on downstream consumer test suites for real-world validation. API Design: Configuration is minimal, point Jest’s transform map at ‘@swc/jest’ with an optional inline options object, and it just works with existing .swcrc files, requiring no new mental model for teams already using SWC elsewhere in their toolchain.