`es/minifier`: .join is not ignoring null values anymore

[skearney414]

Describe the bug

Since #10758 the compiler has broken certain cases of array.join. It no longer ignores null values

Input code

const variable: object | null = {};

const params = [
    'OrderNumber=',
    variable.data?.orderNumber,
    '|AuditNo=',
    variable.data?.auditNo,
    '|JournalType=',
    transactionTypeCode[variable.data.transactionType],
    '|IsPreview=0',
].join('');

console.log(params);

Config

{
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false
    },
    "target": "es5",
    "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": "es6"
  },
  "minify": false,
  "isModule": true
}

Link to the code that reproduces this issue

https://play.swc.rs/?version=1.13.2&code=H4sIAAAAAAAAA3WPTQuCQBRF94L%2FYXajENI6kYhWtbAW7cTFc2aIkXGezIcR2X9Ps4yC3vK%2Bcw9chto60oGRUCmxIljVgjnSE%2B2VIhm53dMwCAP2xFow0NghLcKADEcPhguT%2B6YSJqOLKXy7Eg4O1gl%2BkBdA%2B43n0uX4rwLTe8b36I0Gdbq2Yq44A9oCcxL1mG%2BRi%2BJLk%2FwQ5azb2aMRnRSXbDnayqRGqSNK43kpDg6F52jaG6cPnmfEwCUBAAA%3D&config=H4sIAAAAAAAAA32UO5LbMAxA%2Bz2FR3XapMgBtssZODQJytxQhIYAvdbs%2BO4BJfmTNaROwsOP%2BH29HQ7dB7nu9%2BFLPuVntIWg3P9FQlNmexFJx9MI5Eocuftxo0wNBZsIZtF1IR3b0gM3K6Cfq3qXEAlu6qtsiDmG6Tmgw2EsQPQkE6n4qwNkpv%2FtV1bwswEu9Vl%2BRExg8w4xlkzMDD0UzbHDlOxIYM62KF5aprZEQi1Eg5XBm7HgqPLsI0fMEvOVerDeOPSgoFjAcTyDZiaxxCyTPE95z4w9HGvfz03%2BZg1nm6plJSZc5pZItorXE0ZiE2rWSrjAjRoscC3ud8sYTAGuJb%2FafWDMGz35CyAVSJYo2wE0v7NGkHnasg67ljEHGVmeFC7zrb0yQy9FNTEGpbKtMlA4at0s4KuDVlmnpbPijfJR9GAgBJkVxTV9RnYnLWjbcgwKkP7aoE3VAsx9Czd4W4gd%2FC6vZH3AVo3B8mmb0jQcMe0EGIBP6HcUpBWM27jIlbiM27xmDzIa4FWVSjN4PQKyAIwmzcfyZTZkPcSj6RMeH2diVbjej%2FBgc%2F%2FY9%2BUOv60K3YC%2BznC98K2%2Fy13%2B1T2Ublf4nngX6c%2FNcg56%2FQckCXpSLQYAAA%3D%3D

SWC Info output

No response

Expected behavior

In 1.12.7 this was compressed to var _variable_data, _variable_data1, variable = {};
console.log([
'OrderNumber=',
null == (_variable_data = variable.data) ? void 0 : _variable_data.orderNumber,
'|AuditNo=',
null == (_variable_data1 = variable.data) ? void 0 : _variable_data1.auditNo,
'|JournalType=',
transactionTypeCode[variable.data.transactionType],
'|IsPreview=0'
].join('')); which handled null values correctly

Actual behavior

var _variable_data, _variable_data1, variable = {};
console.log("OrderNumber=" + (null == (_variable_data = variable.data) ? void 0 : _variable_data.orderNumber) + "|AuditNo=" + (null == (_variable_data1 = variable.data) ? void 0 : _variable_data1.auditNo) + "|JournalType=" + transactionTypeCode[variable.data.transactionType] + "|IsPreview=0");

Version

1.13.2

Additional context

No response

[kdy1]

@claude Resolve this issue

[claude]

Claude finished @kdy1's task —— View job • claude/issue-10936-20250724-1343 • Create PR ➔

---

Task List:

• Analyze the issue and understand the problem
• Search for the join optimization code in the minifier
• Understand the current implementation
• Implement a fix to handle null/undefined values
• Add test cases to prevent regression
• Run tests and verify the fix
• Commit and push changes

Summary

Fixed the issue where the minifier was incorrectly optimizing array.join('') to string concatenation when the array contained optional chaining or member expressions that could produce undefined values.

The problem was that the optimization converted undefined values to the string "undefined" instead of an empty string (which is the correct behavior of array.join()).

Changes Made

1. Updated the safety check in /crates/swc_ecma_minifier/src/compress/pure/misc.rs:

   • Added checks for Expr::OptChain and Expr::Member expressions
   • These expressions can produce undefined values, so the optimization is now prevented when they're present

2. Added test cases in /crates/swc_ecma_minifier/tests/fixture/issues/10936/:

   • Tests the original issue with optional chaining
   • Tests literal null and undefined values (which should still be optimized)
   • Tests various forms of optional chaining and member expressions

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