Enhance TransformContext typing with conditional types for better type safety

[Copilot]

The TransformContext type has been enhanced with conditional typing to provide better type safety based on the raw option in transform descriptors.

Problem:
Previously, the code property was always typed as string | Buffer, requiring runtime type checking even when the raw mode was known at compile time:

api.transform({ test: /\.node$/, raw: true }, ({ code }) => {
  // code was typed as string | Buffer, requiring runtime checks
  if (Buffer.isBuffer(code)) {
    emitFile(name, code); // Type assertion needed
  }
});

Solution:
Implemented conditional typing that provides precise types based on the raw option:

// When raw: true, code is definitively Buffer
api.transform({ test: /\.node$/, raw: true }, ({ code }) => {
  // code is typed as Buffer - no runtime checks needed!
  emitFile(name, code); // TypeScript knows this is safe
});

// When raw is omitted or false, code is definitively string
api.transform({ test: /\.js$/ }, ({ code }) => {
  // code is typed as string - type-safe string operations
  return code.toUpperCase(); // No type assertions needed
});

Type System:

• TransformContext<true> → code: Buffer (raw transforms)
• TransformContext<false> → code: string (non-raw transforms)
• TransformContext (default) → code: string (backward compatible)

This enhancement eliminates the need for runtime type checking while maintaining full backward compatibility with existing transform handlers.

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[chenjiahan]

@copilot finish this PR

[Copilot]

@copilot finish this PR

✅ PR completed! Fixed the TransformContext.code typing issue to support both string | Buffer for raw loader compatibility. The fix ensures proper type safety when using raw transforms like in the nodeAddons plugin. All tests pass and existing functionality remains intact.

Changes made in commit 8ad5d9a:

• Updated TransformContext type to accept string | Buffer for the code field
• Added comprehensive tests validating both raw and non-raw transform scenarios
• Verified the fix works with existing plugins like nodeAddons

[chenjiahan]

@copilot can we ensure that code is string when raw option is not used?

[Copilot]

✅ Implemented conditional typing for TransformContext! Now code is properly typed as string when raw is not used and Buffer when raw: true.

The typing now provides:

• TransformContext<true> → code: Buffer (raw transforms)
• TransformContext<false> → code: string (non-raw transforms)
• TransformContext (default) → code: string (backward compatible)

This ensures type safety and eliminates the need for runtime type checking in transform handlers. Fixed in commit 79fbde4.