n8n-io/n8n · error · ExpressionExtensionError

intersection(): expected array arg, e.g. .intersection([1, 2

Error message

intersection(): expected array arg, e.g. .intersection([1, 2, 3, 4])

What it means

The `.intersection([arr])` array extension returns the set-intersection of the source array and the supplied argument. It throws `ExpressionExtensionError` when the first extra argument fails `Array.isArray(...)`, because the implementation immediately calls `.findIndex(...)` on it (which would otherwise crash). The error message shows the expected call shape.

Source

Thrown at packages/@n8n/expression-runtime/src/extensions/array-extensions.ts:302

	const [others] = extraArgs;
	if (!Array.isArray(others)) {
		throw new ExpressionExtensionError(
			'difference(): expected array arg, e.g. .difference([1, 2, 3, 4])',
		);
	}
	const newArr: unknown[] = [];
	for (const v of value) {
		if (others.findIndex((w) => isEqual(w, v)) === -1) {
			newArr.push(v);
		}
	}
	return unique(newArr, []);
}

function intersection(value: unknown[], extraArgs: unknown[][]): unknown[] {
	const [others] = extraArgs;
	if (!Array.isArray(others)) {
		throw new ExpressionExtensionError(
			'intersection(): expected array arg, e.g. .intersection([1, 2, 3, 4])',
		);
	}
	const newArr: unknown[] = [];
	for (const v of value) {
		if (others.findIndex((w) => isEqual(w, v)) !== -1) {
			newArr.push(v);
		}
	}
	for (const v of others) {
		if (value.findIndex((w) => isEqual(w, v)) !== -1) {
			newArr.push(v);
		}
	}
	return unique(newArr, []);
}

function append(value: unknown[], extraArgs: unknown[][]): unknown[] {

View on GitHub (pinned to 5ac6606e81)

Solutions

1. Wrap the argument in an array literal: `.intersection(['urgent'])`.
2. Confirm the upstream field is actually an array (coerce in a Code node with `Array.isArray` / `split(',')`) before calling `.intersection()`.
3. If you only need to test membership of a single value, use `.includes(value)` instead.

Example fix

// before
{{ $json.tags.intersection('urgent') }}
// after
{{ $json.tags.intersection(['urgent']) }}

// In a Code node before the expression runs:
const other = $json.other;
if (!Array.isArray(other)) {
  throw new Error('intersection(): other must be an array, got ' + typeof other);
}
return $json;

const isUnknownArray = (v: unknown): v is unknown[] => Array.isArray(v);

When it happens

Trigger: Invoking `.intersection()` with a scalar, object, or undefined value: `{{ $json.tags.intersection('urgent') }}`, `{{ $json.ids.intersection(5) }}`, or `{{ $json.x.intersection($json.missing) }}` where `$json.missing` resolves to undefined.

Common situations: Forgetting to wrap the argument in `[...]`; pulling a single value out of an upstream node and passing it bare; referencing a field that is a comma-separated string instead of a parsed array.

Related errors

• keys and values must be arrays
• keys and values not of equal length
• removeFieldsContaining(): expected non-empty string arg
• argument of keepFieldsContaining must be a non-empty string
• Filter operator "${operator}" on key "${key}" requires a non

AI-assisted analysis of n8n-io/n8n@5ac6606e81 (2026-08-12). Data as JSON: /api/errors/c534ebd02ce23f39. Report an issue: GitHub.