sindresorhus/is · error · TypeError

Expected value which is `plain object`, received value of ty

Error message

Expected value which is `plain object`, received value of type `${is(value)}`.

What it means

Thrown by `assert.plainObject()` when the value is not a plain object — one created by `{}`, `new Object()`, or `Object.create(null)`. `isPlainObject` rejects arrays, class instances, Maps, Dates, functions, and primitives, which the looser `assert.object()` would accept. It narrows to `Record<PropertyKey, Value>` for safe key-value access.

Source

Thrown at source/index.ts:1836

		throw new TypeError(message ?? typeErrorMessage('Object', value));
	}
}

export function assertObservable(value: unknown, message?: string): asserts value is ObservableLike {
	if (!isObservable(value)) {
		throw new TypeError(message ?? typeErrorMessage('Observable', value));
	}
}

export function assertOddInteger(value: number, message?: string): asserts value is number {
	if (!isOddInteger(value)) {
		throw new TypeError(message ?? typeErrorMessage('odd integer', value));
	}
}

export function assertPlainObject<Value = unknown>(value: unknown, message?: string): asserts value is Record<PropertyKey, Value> {
	if (!isPlainObject(value)) {
		throw new TypeError(message ?? typeErrorMessage('plain object', value));
	}
}

export function assertPositiveInteger(value: unknown, message?: string): asserts value is number {
	if (!isPositiveInteger(value)) {
		throw new TypeError(message ?? typeErrorMessage('positive integer', value));
	}
}

export function assertPositiveNumber(value: unknown, message?: string): asserts value is number {
	if (!isPositiveNumber(value)) {
		throw new TypeError(message ?? typeErrorMessage('positive number', value));
	}
}

export function assertPrimitive(value: unknown, message?: string): asserts value is Primitive {
	if (!isPrimitive(value)) {
		throw new TypeError(message ?? typeErrorMessage('primitive', value));

View on GitHub ↗ (pinned to 7821031c66)

Solutions

If the value is an array of entries or a Map, convert with `Object.fromEntries(value)` before asserting.
If class instances should be accepted, use the looser `assert.object()` instead.
Spread instances into plain objects at the boundary: `{...instance}` — noting this drops methods and prototype.
Check the API contract — a top-level JSON array vs object mismatch means the producer changed shape.

Example fix

// before
const options = new Map([['depth', 2]]);
assert.plainObject(options); // throws: received 'Map'
// after
const options = Object.fromEntries(new Map([['depth', 2]]));
assert.plainObject(options);

Defensive patterns

Strategy: type-guard

Validate before calling

const proto = value === null || typeof value !== 'object' ? undefined : Object.getPrototypeOf(value);
if (typeof value !== 'object' || value === null || (proto !== null && proto !== Object.prototype)) {
  throw new TypeError('Expected a plain object (created by {} or Object.create(null))');
}

Type guard

function isPlainObject(value: unknown): value is Record<string, unknown> {
  if (typeof value !== 'object' || value === null) return false;
  const proto = Object.getPrototypeOf(value);
  return proto === null || proto === Object.prototype;
}

Try / catch

try {
  ow(value, ow.object.plain);
} catch (error) {
  if (error instanceof ArgumentError) {
    throw new TypeError(`Options must be a plain object literal: ${error.message}`);
  }
  throw error;
}

Prevention

Class instances, Maps, arrays, and Dates are objects but not plain objects — spread into a literal ({...instance}) if only data is needed
Objects crossing realm boundaries (iframes, vm, some structuredClone paths) can fail prototype identity checks
Pass options as object literals at the call site rather than reusing framework-constructed objects
JSON.parse output is plain and safe for plain-object parameters

When it happens

Trigger: Calling `assert.plainObject(value)` with an array (a common surprise — arrays are objects but not plain), a class instance, a Map, null, undefined, or a value deserialized into a custom prototype.

Common situations: API returning a JSON array when an object was expected (or vice versa); passing a class-based model/ORM entity where a plain options bag is expected; structuredClone or cross-realm objects with unexpected prototypes; a Map used as a dictionary instead of a plain object.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/457b7d4d748b39a5.json. Report an issue: GitHub ↗.