sindresorhus/is · error · TypeError

Expected value which is `Map`, received value of type `${is(

Error message

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

What it means

Thrown by `assertMap()` (source/index.ts:1705) when `isMap()` fails; it requires the object type tag to be exactly 'Map'. Plain objects, WeakMaps, and Map-serialized-to-JSON values all fail, since the library refuses to treat lookalike structures as real Maps.

Source

Thrown at source/index.ts:1707

		throw new TypeError(message ?? typeErrorMessage('Int8Array', value));
	}
}

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

export function assertIterable<T = unknown>(value: unknown, message?: string): asserts value is Iterable<T> {
	if (!isIterable(value)) {
		throw new TypeError(message ?? typeErrorMessage('Iterable', value));
	}
}

export function assertMap<Key = unknown, Value = unknown>(value: unknown, message?: string): asserts value is Map<Key, Value> {
	if (!isMap(value)) {
		throw new TypeError(message ?? typeErrorMessage('Map', value));
	}
}

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

export function assertNativePromise<T = unknown>(value: unknown, message?: string): asserts value is Promise<T> {
	if (!isNativePromise(value)) {
		throw new TypeError(message ?? typeErrorMessage('native Promise', value));
	}
}

export function assertNegativeInteger(value: unknown, message?: string): asserts value is number {
	if (!isNegativeInteger(value)) {
		throw new TypeError(message ?? typeErrorMessage('negative integer', value));

View on GitHub ↗ (pinned to 7821031c66)

Solutions

Reconstruct after deserialization: `new Map(Object.entries(parsedObject))` or `new Map(pairsArray)`.
Serialize Maps explicitly (`[...map.entries()]`) so the receiving side can rebuild them.
If a plain-object dictionary is acceptable for the use case, assert `assertPlainObject()` instead and access via key lookup.

Example fix

// before
const cache = JSON.parse(raw);
assertMap(cache); // throws — JSON has no Map
// after
const cache = new Map(Object.entries(JSON.parse(raw)));
assertMap(cache);

Defensive patterns

Strategy: type-guard

Validate before calling

if (!(value instanceof Map)) {
  value = new Map(Object.entries(value ?? {})); // if a plain object is acceptable input
}

Type guard

const isMap = (v: unknown): v is Map<unknown, unknown> => v instanceof Map;

Prevention

Plain objects and JSON-parsed data are never Maps — convert with new Map(Object.entries(obj)) at the boundary
JSON.parse cannot produce a Map; deserialize explicitly if your wire format encodes map data
WeakMap does not pass an is.map check — they are distinct types

When it happens

Trigger: Calling `assertMap()` with a plain object used as a dictionary, a WeakMap (tag 'WeakMap'), an array of [key, value] pairs, or the result of `JSON.parse` on serialized Map data (Maps serialize to `{}`).

Common situations: The JSON round-trip is the top cause: `JSON.stringify(new Map())` yields '{}', so anything persisted/transported loses Map-ness. Also codebases mixing object-dictionaries with Maps, or receiving `Object.fromEntries` output where a Map is expected.

Related errors

AI-assisted analysis of sindresorhus/is@7821031c66 (2026-07-31). Data as JSON: /data/errors/2b865b5f49678e92.json. Report an issue: GitHub ↗.