oxc-project/oxc · warning · OxcDiagnostic

Unexpected use of comma operator

Error message

Unexpected use of comma operator

What it means

oxlint's port of ESLint `no-sequences`. It flags the comma operator, which evaluates both operands and returns the last — a construct that hides side effects and is frequently a typo for semicolons or for array literals. With `allowInParentheses: false` (oxlint default), even explicitly parenthesized sequences like `(a, b)` are flagged; setting it true permits sequences wrapped in parentheses.

Source

Thrown at crates/oxc_linter/src/rules/eslint/no_sequences.rs:14

use oxc_ast::AstKind;
use oxc_diagnostics::OxcDiagnostic;
use oxc_macros::declare_oxc_lint;
use oxc_span::{GetSpan, Span};
use schemars::JsonSchema;
use serde::Deserialize;

use crate::{
    AstNode, ast_util::outermost_paren_parent, context::LintContext, rule::DefaultRuleConfig,
    rule::Rule,
};

fn no_sequences_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Unexpected use of comma operator")
        .with_help("Do not use the comma operator. If you intended to write a sequence, wrap it in parentheses.")
        .with_label(span)
}

#[derive(Debug, Clone, JsonSchema, Deserialize)]
#[serde(rename_all = "camelCase", default, deny_unknown_fields)]
pub struct NoSequences {
    /// If this option is set to `false`, this rule disallows the comma operator
    /// even when the expression sequence is explicitly wrapped in parentheses.
    allow_in_parentheses: bool,
}

impl Default for NoSequences {
    fn default() -> Self {
        Self { allow_in_parentheses: true }
    }
}

View on GitHub (pinned to e1e7af627c)

Solutions

1. Split into separate statements: `a(); const y = b;`.
2. If both results are needed, use an array `const [r1, r2] = [a(), b];` or chaining with `&&`.
3. For deliberate short sequences, keep them in a `for` header or set `"allowInParentheses": true`.

Example fix

// before
const y = (sideEffect(), value);

// after
sideEffect();
const y = value;

// Find comma-operator sequences outside for-headers
function sequenceOutsideFor(src) {
  const noFor = src.replace(/for\s*\([^)]*\)/g, '');
  return /\((?:[^()]*,)+[^()]*\)(?!\s*\()/.test(noFor) && /,/.test(noFor.replace(/\[[^\]]*\]/g, ''));
}

When it happens

Trigger: `x = (1, 2);`, `const y = (a(), b);`, `if ((a, b)) ...`, `return 1, 2;` — the rule inspects SequenceExpression nodes found via `outermost_paren_parent`. Sequence expressions in `for` headers are the conventional tolerated location.

Common situations: Minified/hand-compressed code pasted into a repo; obfuscated snippets; accidental comma where a semicolon or `&&` was intended.

Related errors

• Returned expression contains an assignment.
• Empty array binding pattern
• Empty object binding pattern
• `debugger` statement is not allowed
• Variables should not be deleted

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/86a491e54ed10856. Report an issue: GitHub.