`deno check` ignores JSX compilerOptions from `deno.json` when a `tsconfig.json` is present

[quentinadam]

Versions

• Deno version: 2.7.11
• OS: macOS (Darwin)
• TypeScript version (bundled): 5.9.2

Description

When a project has valid JSX-related compilerOptions (jsx and jsxImportSource) in deno.json, running deno check works correctly. However, if a tsconfig.json file exists in the project root (even an empty one), deno check fails with JSX-related TypeScript errors.

It appears that the presence of a tsconfig.json causes Deno to ignore the JSX compiler options defined in deno.json. This issue seems to be specifically related to JSX configuration (jsx and jsxImportSource options).

Reproduction

Minimal reproduction repository: https://github.com/quentinadam/deno-tsconfig-issue

Steps to reproduce:

1. Clone the repository:

   git clone https://github.com/quentinadam/deno-tsconfig-issue.git
   cd deno-tsconfig-issue

2. Run deno check:

   deno check Component.tsx

   Result: Fails with errors

3. Remove or rename tsconfig.json:

   mv tsconfig.json tsconfig.json.bak
   deno check Component.tsx

   Result: Succeeds

Docker reproduction (clean environment):

docker run --rm -v "$(pwd)":/app -w /app denoland/deno:latest sh -c "
  echo '--- With tsconfig.json ---' &&
  deno check Component.tsx;
  echo '--- Without tsconfig.json ---' &&
  mv tsconfig.json tsconfig.json.bak &&
  deno check Component.tsx;
  mv tsconfig.json.bak tsconfig.json
"

This confirms the issue in a fresh Deno environment with no local cache.

Project structure:

deno.json:

{
  "imports": {
    "preact": "npm:preact@^10.29.1"
  },
  "compilerOptions": {
    "jsx": "react-jsx",
    "jsxImportSource": "preact"
  }
}

tsconfig.json:

{}

Component.tsx:

export function Component() {
  return <div>Hello world</div>;
}

Error output

Check Component.tsx
TS7026 [ERROR]: JSX element implicitly has type 'any' because no interface 'JSX.IntrinsicElements' exists.
  return <div>Hello world</div>;
         ~~~~~
    at file:///[...]/Component.tsx:2:10

TS2875 [ERROR]: This JSX tag requires the module path 'preact/jsx-runtime' to exist, but none could be found. Make sure you have types for the appropriate package installed.
  return <div>Hello world</div>;
         ~~~~~~~~~~~~~~~~~~~~~~
    at file:///[...]/Component.tsx:2:10

Found 3 errors.

error: Type checking failed.

Expected behavior

According to the Deno TSConfig migration documentation:

If a parent deno.json contains compilerOptions, that will take precedence over any TSConfig.

Based on this documented behavior, deno check should succeed because the compilerOptions in deno.json should take precedence over the empty tsconfig.json. However, this is not what happens in practice for JSX-related options.

[verysleepylemon]

When both deno.json and tsconfig.json are present, Deno's TypeScript compiler uses tsconfig.json for compilerOptions and the JSX settings in deno.json are silently ignored. This is because Deno passes the resolved tsconfig.json directly to the TypeScript compiler, which then takes precedence.

The fix: put your JSX compiler options in tsconfig.json directly:

// tsconfig.json
{
  "compilerOptions": {
    "jsx": "react-jsx",
    "jsxImportSource": "preact",
    "lib": ["dom", "dom.iterable", "esnext"],
    "strict": true
  }
}

And keep deno.json for Deno-specific config (imports, tasks, lint, fmt):

// deno.json
{
  "imports": {
    "preact": "npm:preact@10",
    "preact/jsx-runtime": "npm:preact@10/jsx-runtime"
  },
  "tasks": {
    "dev": "deno run --allow-net main.ts"
  }
}

Why this happens: deno.json's compilerOptions key is a convenience that works when there is no tsconfig.json. Once Deno finds a tsconfig.json, it delegates entirely to that file for compiler options. The two are not merged.

If you need both files and want to keep deno.json as the source of truth, you can use deno.json's compilerOptions and remove tsconfig.json — or rename it to a path not auto-discovered (e.g., tsconfig.base.json) and reference it explicitly with deno check --config tsconfig.base.json only when needed.

This behavior is arguably a documentation gap — the precedence rules between the two config files are not prominently documented.

[quentinadam]

Thanks for the explanation!

However, I'm still experiencing some inconsistent behavior that suggests the two files are not being handled with a clear precedence rule. It seems that some compilerOptions keys are taken from tsconfig.json while others are taken from deno.json, rather than one file being fully discarded when the other is present.

For example, with the noUncheckedIndexedAccess option:

Test setup:

// Component.tsx
const array = [1, 2, 3, 4, 5];
const _value: number = array[0]; // Should error if noUncheckedIndexedAccess is enabled

export function Component() {
  return <div>Hello world</div>;
}

Scenario 1: noUncheckedIndexedAccess in tsconfig.json only

// tsconfig.json
{ "compilerOptions": { "noUncheckedIndexedAccess": true } }

// deno.json
{ "compilerOptions": { "jsx": "react-jsx", "jsxImportSource": "preact" }, ... }

Result: JSX errors appear (JSX options from deno.json ignored), but NO error for array[0] assignment (noUncheckedIndexedAccess from tsconfig.json is also ignored).

Scenario 2: noUncheckedIndexedAccess in deno.json only

// tsconfig.json
{}

// deno.json
{ "compilerOptions": { "jsx": "react-jsx", "jsxImportSource": "preact", "noUncheckedIndexedAccess": true }, ... }

Result: JSX errors appear (as expected since tsconfig.json is present), AND the TS2322: Type 'number | undefined' is not assignable to type 'number' error appears (noUncheckedIndexedAccess from deno.json IS applied).

This behavior is inconsistent:

• JSX options (jsx, jsxImportSource) from deno.json are ignored when tsconfig.json is present
• But noUncheckedIndexedAccess from tsconfig.json is ignored, while being applied when set in deno.json

It seems like neither file fully takes precedence - different options are being sourced from different files in an unpredictable way.

I've updated the reproduction repository to reflect this: https://github.com/quentinadam/deno-tsconfig-issue