minify: Construction of class with side-effect free constructor is not treated as dead code

[dmichon-msft]

Describe the bug

Invoking a constructor for a class that only delegates to Object() is a side-effect free operation and should be able to be discarded.

Input code

new class {}

Config

{
  "jsc": {
    "parser": {
      "syntax": "ecmascript",
      "jsx": false
    },
    "target": "es2024",
    "loose": false,
    "minify": {
      "compress": {
        "arguments": false,
        "arrows": true,
        "booleans": true,
        "booleans_as_integers": false,
        "collapse_vars": true,
        "comparisons": true,
        "computed_props": true,
        "conditionals": true,
        "dead_code": true,
        "directives": true,
        "drop_console": false,
        "drop_debugger": true,
        "evaluate": true,
        "expression": false,
        "hoist_funs": false,
        "hoist_props": true,
        "hoist_vars": false,
        "if_return": true,
        "join_vars": true,
        "keep_classnames": false,
        "keep_fargs": true,
        "keep_fnames": false,
        "keep_infinity": false,
        "loops": true,
        "negate_iife": true,
        "properties": true,
        "reduce_funcs": false,
        "reduce_vars": false,
        "side_effects": true,
        "switches": true,
        "typeofs": true,
        "unsafe": false,
        "unsafe_arrows": false,
        "unsafe_comps": false,
        "unsafe_Function": false,
        "unsafe_math": false,
        "unsafe_symbols": false,
        "unsafe_methods": false,
        "unsafe_proto": false,
        "unsafe_regexp": false,
        "unsafe_undefined": false,
        "unused": true,
        "const_to_let": true,
        "pristine_globals": true
      },
      "mangle": false
    }
  },
  "module": {
    "type": "commonjs"
  },
  "minify": false,
  "isModule": true
}

Link to the code that reproduces this issue

https://play.swc.rs/?version=1.15.2&code=H4sIAAAAAAAAA8tLLVdIzkksLlaorgUARnxBwQwAAAA%3D&config=H4sIAAAAAAAAA32US47bMAyG93OKwOsuikFXc4DuegZBsShHqSQaIpWJMcjdS8vOoxM6O5sff1IiKX697XbdkfruY%2Fcln%2FIz2kJQbv9ioSmzPYulgz5Z6ksYuftxpUeakbeRoJkuC%2BnYlgG4qej95%2FuvVdFFRIKrYrWlkIOfHnP2mMYCRA82sUrImiAz%2Fa9fWcHPGXCpj%2FY9YgSbXxBjyYTMMEDRAvcYox0JzMkWJcp8UlsCoZZihpXBmbHgqPLsAgfMkvOZOrDO9OhAQaFAz%2BEEmkxyiSyTXE%2B5T8MO9nUYWp%2B%2FqeFkY7Ws5IRza4mcVol6wEBsfM1aCRe4UYMFrsX9rgzeFOBa8rPuiCFv9OQvgFQgWqJsE2hxm4eXedpS%2B5fKkL2MLE8Kl%2FnWbplhkKKaELxS2bkyUDho3Szgag9zZXvtOCveKB8FBwa8l1lRQtNn4P6gJeVpBPQKkP5ar03VAsztFW7w%2BUG8wL%2FllqwP2OqRLB%2B2KU1pj%2FFFggR8QPfCQVrBuI2LbInzuM1rdiCjAU51qdTA8xKQB8BoYtuXT7Mhz0MimiHi%2Fr4mVofLbQ8nm4f7e19W8dvq0CV0tcF1yc%2F9nVeztCNhPlJ397yu4tvpu0B%2FrvKW%2BfIP0%2BWeLDUGAAA%3D

SWC Info output

No response

Expected behavior

new class {} gets removed from the output

Actual behavior

new class {} remains in the output

Version

1.15.2

Additional context

No response

[kdy1]

@claude Fix it

[claude]

Claude finished @kdy1's task —— View job • claude/issue-11320-20251121-0142 • Create PR ➔

---

Fixed the minifier issue ✅

Summary

Successfully implemented a fix to remove side-effect free class instantiations like new class {} during minification.

What was done

• Analyzed the minifier codebase to understand dead code elimination
• Located where class instantiation side effects are analyzed
• Implemented fix in misc.rs:2178-2196 that detects side-effect free class constructors
• Created comprehensive unit tests in tests/terser/compress/dead_code/issue_11320/
• Ran cargo fmt to format code
• Verified all tests pass ✅
• Committed and pushed changes

Implementation details

The fix adds a check in the pure optimizer's ignore_return_value method that:

• Detects when NewExpr has a ClassExpr as its callee
• Uses the existing class_has_side_effect() function to determine if the class is pure
• Removes the class instantiation if it has no side effects, keeping only arguments with side effects

The implementation correctly handles:

• Empty classes: new class {} → removed
• Classes with side-effect arguments: new class {}(foo()) → foo()
• Classes with super class side effects: kept
• Classes with computed property keys: kept
• Classes with field initializers: kept

--- | Branch