In this article, we analyze Istanbul usage in tRPC source code. I found this comment — istanbul ignore if. This hints that tRPC uses Istanbul.js, a tool that makes JavaScript test coverage simple.

This one took me a while to figure out that tRPC repository uses @vitest/coverage-istanbul, I was initially looking to see if there’s any scripts related to test defined in packages/clients but there were none.

After searching for istanbul across the codebase, that is when I saw Istanbul word in vitest.config.ts test scripts are defined in the root level’s package.json.

"test": "turbo codegen-tests && conc -c \"green,blue\" \"vitest run\" \"pnpm -F tests test-run:tsc\"",
"test-ci": "turbo codegen-tests && conc \"CI=true vitest run - coverage\" \"pnpm -F tests test-run:tsc\"",
"test-watch": "vitest",

Below is coverage object picked from vitest.config.ts:

coverage: {
 provider: 'istanbul',
 include: ['**/src/**'],
 exclude: [
 '**/www/**',
 '**/examples/**',
 // skip codecov for experimental features
 // FIXME: delete me once they're stable
 '**/next/src/app-dir/**',
 '**/server/src/adapters/next-app-dir/**',
 ],
},

Vitest supports another provider as well, it is ‘v8’. By default, provider is set to v8.

Let’s what happens when test script is run:

test script:

"test": "turbo codegen-tests && conc -c \"green,blue\" \"vitest run\" \"pnpm -F tests test-run:tsc\"",

tRPC uses Turbo. Turbo is an incremental bundler and build system optimized for JavaScript and TypeScript, written in Rust.

turbo codegen-tests:

codegen-tests is a command defined in turbo.json and when you run this, it executes codegen-tests scripts defined in the packages. This is a monorepo setup.

codegen-scripts in packages:
- client/package.json
- next/package.json
- react-query/package.json
- server/package.json

conc -c

conc is a short alias for concurrently. Checkout concurrrently.

Below is an example usage of concurrently.

concurrently "command1 arg" "command2 arg"
(or)
conc "command1 arg" "command2 arg"

tRPC uses this below command:

conc -c \"green,blue\" \"vitest run\" \"pnpm -F tests test-run:tsc\"

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References:

1. https://github.com/trpc/trpc/blob/next/packages/client/src/links/httpBatchLink.ts#L91C12-L91C30

2. https://github.com/gotwarlost/istanbul

3. https://istanbul.js.org/

4. https://github.com/istanbuljs

5. https://github.com/trpc/trpc/blob/d603d860a3aeb12bbf6e836abd8c5a30c7b5d7a5/vitest.config.ts#L45