Implement optional chaining.

[xeioex]

This closes #835 feature request on Github.

[Copilot]

Pull request overview

Adds runtime and parsing support for JavaScript optional chaining (?.) in njs, addressing the behavior reported in issue #835 (e.g. f?.() / obj?.prop short-circuiting on null/undefined).

Changes:

• Introduces a new VM opcode (NJS_VMCODE_OPTIONAL_CHAIN) to implement optional-chain short-circuiting at runtime.
• Updates the parser and bytecode generator to build/emit optional-chaining AST + vmcode (including ?.() call forms).
• Adds a comprehensive set of unit tests covering property access, calls, delete semantics, and interactions with ??.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/test/njs_unit_test.c	Adds unit tests for optional chaining behavior across many forms/corner cases.
src/njs_vmcode.h	Adds NJS_VMCODE_OPTIONAL_CHAIN opcode to the VM opcode enum.
src/njs_vmcode.c	Implements interpreter handling for the optional-chain opcode and wires it into the dispatch table.
src/njs_parser.c	Extends parsing to recognize/construct optional-chaining AST nodes and delete behavior adjustments.
src/njs_lexer.h	Introduces AST token types for optional chaining nodes (OPTIONAL_CHAIN, OPTIONAL_CHAIN_REF).
src/njs_generator.c	Generates bytecode for optional chaining, including short-circuit jump patching and this handling in method-call cases.
src/njs_disassembler.c	Adds disassembly formatting for the new optional-chain opcode.

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Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

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[VadimZhestikov]

Do we need add more testcases, like:

for run-time:

  o[a].m?.()
  o?.[expr].m?.()      or mixed nested optional segments

for parser:

  a ? .b : c (weird spacing) does not accidentally parse as optional chaining.
  a?.b : c properly errors (because : would be nonsense there).
  a ? b?.c : d still parses as ternary with optional chaining inside.

negative-syntax tests:

o?.a = 1 (optional chaining is not a valid assignment target)
o?.a++ / ++o?.a
new o?.() (optional call with new is invalid in JS)
o?.a().b is allowed, but o?.a?.().b shapes should be validated if supported

[xeioex]

@VadimZhestikov

1. I reworked the patch set by adding preparatory patches which minimize the code for new functionality.
   They are mostly mechanical and pretty regular.
2. I also reworked Implement optional chaining be reducing the coupling between parser and generator. Now generator part is mostly straightforward (no complex logic).
3. I added more tests based on your examples and found 2 bugs for expressions like o[a].m?.()
   and o?.[expr].m?.() they pass now.

[VadimZhestikov]

Looks good