oxc-project/oxc · warning · OxcDiagnostic

Do not call `.add()` immediately after initializing a Set.

Error message

Do not call `.add()` immediately after initializing a Set.

What it means

Diagnostic from the oxlint rule `unicorn/no-immediate-mutation` (Set arm, category: pedantic). `const set = new Set([1, 2]); set.add(3);` builds the Set in two statements; the element can be passed in the constructor's initializer array instead. Fires for the global `Set` and `WeakSet` when `add()` with exactly one non-spread argument (not referencing the set itself) is the statement immediately after the declaration.

Source

Thrown at crates/oxc_linter/src/rules/unicorn/no_immediate_mutation.rs:39

    .with_label(span)
}

fn object_assign_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Do not call `Object.assign()` immediately after initializing an object.")
        .with_help("Move the properties from `Object.assign()` into the object initializer.")
        .with_label(span)
}

fn object_property_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn(
        "Do not assign a property immediately after initializing an object literal.",
    )
    .with_help("Move the property into the object initializer.")
    .with_label(span)
}

fn set_add_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Do not call `.add()` immediately after initializing a Set.")
        .with_help("Add the element to the Set initializer array.")
        .with_label(span)
}

fn map_set_diagnostic(span: Span) -> OxcDiagnostic {
    OxcDiagnostic::warn("Do not call `.set()` immediately after initializing a Map.")
        .with_help("Add the entry to the Map initializer array.")
        .with_label(span)
}

#[derive(Debug, Default, Clone)]
pub struct NoImmediateMutation;

declare_oxc_lint!(
    /// ### What it does
    ///
    /// Disallows mutating a variable immediately after initialization.
    ///

View on GitHub (pinned to e1e7af627c)

Solutions

1. Fold the element into the initializer: `const seen = new Set([url]);`
2. For many elements spread an iterable: `new Set([...urls, extra])`
3. If membership depends on runtime state not expressible in the initializer, keep `add()` and suppress inline

Example fix

// before
const seen = new Set();
seen.add(url);

// after
const seen = new Set([url]);

When it happens

Trigger: `const seen = new Set(); seen.add(url);` or `const weak = new WeakSet([a]); weak.add(b);` — declaration via `new Set`/`new WeakSet` (any arguments, including none) directly followed by `<sameVar>.add(x)`. `add(...spread)`, zero or multiple args, and `set.add(set.size)` pass.

Common situations: Dedupe/visited sets built incrementally; the `const s = new Set(); s.add(x)` idiom carried over from older code; WeakSets used as privacy markers.

Related errors

• Do not call `.set()` immediately after initializing a Map.
• The {expr_type} is useless
• Use `Set#size` instead of converting a `Set` to an array and
• `VirtualFree` failed during cleanup: {err}
• Tried to get an allocator from an empty `FixedSizeAllocatorP

AI-assisted analysis of oxc-project/oxc@e1e7af627c (2026-08-20). Data as JSON: /api/errors/2f0b3668cd791530. Report an issue: GitHub.