swc-project/swc · error · TypeError

Object is not iterable.

Error message

Object is not iterable.

What it means

`_ts_values` is the iteration helper emitted for `for...of` (and iterable destructuring) when TypeScript-style downlevel iteration is compiled by SWC. It accepts real iterables (`typeof o[Symbol.iterator] === 'function'`) or array-likes (`typeof o.length === 'number'`); anything else triggers `TypeError: Object is not iterable.`

Source

Thrown at crates/swc_ecma_transforms_base/src/helpers/generated/_ts_values.rs:25

    local: "_ts_values",
    import_path: "@swc/helpers/_/_ts_values",
    #[cfg(feature = "inline-helpers")]
    source: r#"function _ts_values(o) {
    var s = typeof Symbol === "function" && Symbol.iterator, m = s && o[s], i = 0;
    if (m) {
        return m.call(o);
    }
    if (o && typeof o.length === "number") {
        return {
            next: function() {
                if (o && i >= o.length) {
                    o = void 0;
                }
                return { value: o && o[i++], done: !o };
            }
        };
    }
    throw new TypeError(s ? "Object is not iterable." : "Symbol.iterator is not defined.");
}
"#,
    #[cfg(feature = "inline-helpers")]
    deps: super::HelperBitmap::from_bits(0x00000001000000000000000000000000),
};

#[cfg(feature = "inline-helpers")]
pub fn stmts() -> &'static [swc_ecma_ast::Stmt] {
    static STMTS: once_cell::sync::Lazy<Vec<swc_ecma_ast::Stmt>> =
        once_cell::sync::Lazy::new(|| super::super::parse(DEF.source, DEF.import_path));
    &STMTS
}

View on GitHub (pinned to 5176682b65)

Solutions

1. Use `Object.entries(obj)` / `Object.keys` / `Object.values` for plain-object iteration.
2. Normalize before the loop: `for (const x of Array.from(obj))` when the value is array-like.
3. Add an iterator to your custom class: `*[Symbol.iterator]() { ... }`.
4. Fix the data source to hand the loop an actual array/iterable.

Example fix

// before
for (const item of config) {} // config = { a: 1 }
// TypeError: Object is not iterable.

// after
for (const [key, value] of Object.entries(config ?? {})) {}

// Mirror _ts_values: accept iterables or array-likes before for-of.
const isIterableOrArrayLike = (v) =>
  v != null &&
  (typeof v[Symbol.iterator] === 'function' || typeof v.length === 'number');
const source = await getData();
if (!isIterableOrArrayLike(source)) throw new TypeError(`expected iterable, got ${typeof source}`);
for (const x of source) { /* ... */ }

const isIterableOrArrayLike = <T>(v: unknown): v is Iterable<T> | ArrayLike<T> =>
  v != null &&
  (typeof (v as Iterable<T>)[Symbol.iterator] === 'function' ||
    typeof (v as ArrayLike<T>).length === 'number');

try {
  for (const x of obj) { handle(x); }
} catch (err) {
  if (err instanceof TypeError && /not iterable/.test(err.message)) {
    for (const value of Object.values(obj ?? {})) { handle(value); }
  } else throw err;
}

When it happens

Trigger: `for (const x of obj)` where `obj` is a plain object (no `[Symbol.iterator]`, no `.length`) in output compiled with downlevel iteration helpers — typically TS with target below ES2015 or `downlevelIteration` semantics.

Common situations: Iterating a plain object expecting entries instead of using `Object.entries`; an API changing an array response to an object; DOM/custom collections without iterator support; Map/Set confused with plain objects.

Related errors

• Invalid attempt to destructure non-iterable instance. In ord
• Invalid attempt to spread non-iterable instance. In order to
• Duplicated element (${elements[j].key})
• Cannot destructure ${o}
• Object expected.

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/ecb55ecd40d5df4b. Report an issue: GitHub.