Node Compat

Node Compat Test

Work on Node.js compatibility test $ARGUMENTS.

Step 1: Build and run the test

./x build
./x test-compat $ARGUMENTS

If the test passes, report success and ensure test is specified in

tests/node_compat/config.jsonc.

Step 2: Diagnose the failure

Read the test file to understand what it tests:

# Tests live under tests/node_compat/test/

Use Grep and Read to find the test source, then analyze:

• What Node.js API or behavior is being tested?
• What is the actual error or assertion failure?
• Where is the relevant Deno implementation? Check ext/node/ (polyfills,

ops, internal bindings), runtime/, or cli/.

Read the corresponding Node.js docs and/or source code to understand the

expected behavior.

Step 3: Classify the failure

Determine which category this failure falls into:

A. Fixable bug

The Deno implementation is wrong or incomplete, but can be corrected. This

includes:

• Missing method/property on a polyfill
• Wrong return value or error type
• Missing event emission
• Incorrect argument handling
• Hard-to-fix but still fundamentally implementable behaviors

Action: Fix the implementation (Step 4).

B. Inherent incompatibility

The test relies on Node.js internals or architecture that Deno fundamentally

cannot or will not support:

• internalBinding() calls to Node's C++ layer
• Node.js-specific CLI flags (--inspect, --prof, etc.)
• V8 internals exposed through Node-specific APIs
• Node.js-specific build/addon tooling (node-gyp internals)
• Tests for Node.js's own test infrastructure

Action: Ignore the test with a reason (Step 5).

C. Not worth fixing

The test exercises an edge case or behavior that is technically possible but

provides negligible value:

• Extremely obscure error message wording differences
• Node.js-specific deprecation warnings
• Behavior that no real-world code depends on

Action: Ignore or skip the test with a reason (Step 5).

Step 4: Fix the implementation

If the failure is fixable:

• Locate the relevant code in ext/node/ (or elsewhere).
• Implement the fix. Match Node.js behavior - check Node.js docs and/or source

code, not just what "seems right."

• Use lazy-loaded imports where possible.
• Use primordials for internal JS code to avoid prototype pollution.
• Rebuild the source code with ./x build - this is paramount, changes won't

take effect until you do.

• Re-run the test to verify the fix:

./x test-compat $ARGUMENTS

• Once the test passes, make sure the test is listed in

tests/node_compat/config.jsonc with an empty config:

"category/test-name.js": {}

The entries in config.jsonc are sorted alphabetically within their category.

Place the new entry in the correct position.

Step 5: Skip or ignore the test

If the test cannot or should not be fixed, update

tests/node_compat/config.jsonc.

Ignore (test should never run — inherent incompatibility)

"category/test-name.js": {
  "ignore": true,
  "reason": "Brief, specific explanation of why this can't work in Deno"
}

"reason" must be specified otherwise the lint step will fail!

Platform-specific skip

If the test only fails on certain platforms:

"category/test-name.js": {
  "windows": false
}

Expected failure (test runs but fails with known output)

If you want the test to run but expect a specific failure:

"category/test-name.js": {
  "exitCode": 1,
  "output": "[WILDCARD]specific error message[WILDCARD]",
  "reason": "Brief explanation of why this fails"
}

This is a good middle ground for tests that are generally compatible but have a

specific known issue. If a fix is ever done this assertion will notify the

implementer to update the config.

Writing good reasons

Reasons should be specific and actionable. Good examples:

• "Tests Node.js internal C++ binding (internalBinding('zlib').Zlib) which is

not implemented in Deno"

• "requires deno --interactive flag (not yet implemented)"
• "URL.createObjectURL does not throw ERR_INVALID_ARG_TYPE for non-Blob

arguments"

Bad examples:

• "Not supported" (too vague)
• "Doesn't work" (says nothing)
• "Node-specific" (which part?)

Step 6: Verify

Re-run the test one final time to confirm the outcome matches expectations:

./x test-compat $ARGUMENTS

Config reference

The full schema for config.jsonc entries is in

tests/node_compat/schema.json.

PR title conventions

When opening a PR for node-compat work, use the appropriate prefix:

• test: — when the PR only updates tests/node_compat/config.jsonc to skip,

ignore, or otherwise reclassify tests without changing implementation code.

• fix(ext/node): — when the PR actually fixes the implementation so a

previously failing test now passes (typically changes under ext/node/ plus

enabling the test in config.jsonc).