Compression misplaces function call when variable storing return value is used for logical assignment

[augustjk]

Describe the bug

When a return value of a function is assigned to a let variable, and that variable is used behind a logical assignment, compression tries to remove that intermediate variable and move the function call directly after the logical assignment.

This changes the logic such that before compilation the function would have always been called, but after compilation it's moved behind the logical assignment and will be called conditionally.

Since the function can have a side-effect that should always run, this is incorrect behavior.

Input code

const myArr = [];
// function with side effect
function foo(arr) {
    arr.push('foo');
    return 'foo';
}
let a;

if (Math.random() > 0.5) {
    a = true;
}

// the function call below should always run
// regardless of whether `a` is `undefined`
let b = foo(myArr);

// const seems to keep this line here instead of
// moving it behind the logitcal nullish assignment
// const b = foo(myArr);

a ??= b;

console.log(a);
console.log(myArr);

Config

{
  "jsc": {
    "parser": {
      "syntax": "ecmascript",
      "jsx": false
    },
    "target": "es2022",
    "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,
        "passes": 2
      },
      "mangle": false
    }
  },
  "module": {
    "type": "es6"
  },
  "minify": false,
  "isModule": true
}

Playground link (or link to the minimal reproduction)

https://play.swc.rs/?version=1.3.74&code=H4sIAAAAAAAAA2WQTU7DMBCF9z7F2zXZpGxYRaXiAJygQsoknsQWjl35h6hC3J1xJFokvBrPzHvvs6fgU8Z6e40RJ1zee3U8Yi5%2ByjZ4bDYbJKtZ3VtzCA3F2OJLQY6U3bUk0xxkcGj7vRk5l%2Bixt3r1rRxnUK%2BUndG8UTZdJK%2FD2rR4wVP3fPcSghwLV0nFyIYfKBM5h5Fd2JBMKE6D3Ea3hFh8XY68UNSOU0KYsRkWdcRAA2zCULzm2XrWww4zSlJ9yP5uga4G0%2F4TiXlNyAEfzFchELETHcSMYWWDSUtAFazh0%2FoFVuzYWK93XhcWm4UVvjhnkwGlZBe%2Fss%2BPkH%2FxhPP5hFGquhAcd%2BLTkIz%2B3n%2FXfwDJh74ktAEAAA%3D%3D&config=H4sIAAAAAAAAA32UO3LjMAyGe5%2FCozrFjostcoDtcgYOTYIyvXxoCNCxJuO7L0TJjjeG1En48AMkAOJrt993ZzTd%2B%2F6LP%2Fln0AWhPP7ZgmMifWVLByZqNMUP1L3d6Rkn5HRAaKbbTDrSpQdqKjz8OhwWRRdyRrgrFlv0ybvxOafJcSiA%2BGRjK4esERLh%2F%2FqFlfw5ASr12X7MOYBOG0RpVD4R9FCkwCaHoAcEddFFiDKdVBePWUoxwUpg1VDyIPJkPfmcOOcrtaCtMtmCgHwBQ%2F4CkoxzsSwhX0%2B4T8MWjrXvW59%2FqOGiQ9Uk5IRrawmfVoh6yh5JuZqkEs5wpQYzXIr7U%2BmdKkC1pFfdOfu00pO%2FAFyBoBGTjiDFbR6O52lN7TaVPjkeWRoFzvMt3TJBz0VV3juhslNloJCXulnAVgNTZY10nAWvlA%2B9BQXO8awIofHTkzlJSWkcIDsBcH%2B1k6ZqBurxClf49CA28B%2B%2BJckDtnhETad1imM85rCRIAKdst1w4FZQXseFt8R1WOc1WeDRACu6VGzgdQnwA6CsQtuXL7PBz4Mjqj7ko7gmBp7z1sTDYrs9lnPUqf9eAvN%2B3i0OXcy2Nrhs%2Fqnp877%2B3X073Vfz4zadx4%2B7cjrJ7vYPdLfRTEUGAAA%3D

SWC Info output

No response

Expected behavior

In above input, the output should keep b = foo(myArr) like how it does when it's declared with const instead of let.

Actual behavior

No response

Version

1.3.74

Additional context

No response

[kdy1]

Seems like this occurs only if someone tries to minify using transform() instead of minify().

[kdy1]

Strange thing is that

const myArr = [];
        function foo(arr) {
            arr.push('foo');
            return 'foo';
        }
        let a;
        if (Math.random() > ) {
            a = true;
        }
        let b = foo(myArr);
        a ??= b;
        console.log(a);
        console.log(myArr);

is Start of the input when it's invoked via transform(), but minifier does not break it if I feed the Start to it directly.

So... my guess is that it's related to span hygiene.

[swc-bot]

This closed issue has been automatically locked because it had no new activity for a month. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.