jestjs/jest · error · TypeError

You must provide an array to

Error message

You must provide an array to ${this.toString()}, not '${typeof this.sample}'.

What it means

arrayContaining / arrayNotContaining build an ArrayContaining matcher whose asymmetricMatch checks that this.sample is an array before testing containment. Although TypeScript types the sample as Array<unknown>, at runtime (plain JS, or a cast) a non-array sample triggers this TypeError at match time. The check exists so the failure is reported clearly instead of producing a confusing `.every` error.

Solutions

Pass an array literal: `expect.arrayContaining([item1, item2])`.
If you intended to match an object's subset, use `expect.objectContaining({...})` instead.
Verify the runtime shape of the value passed (log it / assert Array.isArray) before constructing the matcher.

Example fix

// before
expect(result).toEqual(expect.arrayContaining({ id: 1 })); // object, not array

// after
expect(result).toEqual(expect.arrayContaining([{ id: 1 }]));
// or for object subset:
expect(result).toEqual(expect.objectContaining({ id: 1 }));

Defensive patterns

Strategy: type-guard

Validate before calling

function arrayContainingSafe(sample) {
  if (!Array.isArray(sample)) {
    throw new TypeError('arrayContaining requires an array');
  }
  return expect.arrayContaining(sample);
}

Type guard

function isUnknownArray(v: unknown): v is unknown[] {
  return Array.isArray(v);
}

const m = isUnknownArray(sample) ? expect.arrayContaining(sample) : expect.anything();

Prevention

Always wrap arrayContaining samples in [...].
Use objectContaining for object subsets, arrayContaining for array contents.
Log/assert the runtime shape of dynamic samples before building matchers.

When it happens

Trigger: Calling `expect.arrayContaining(value)` where value is not an array — e.g. an object, a single primitive, or undefined. The error surfaces when the matcher actually runs (during toEqual/toStrictEqual), not at construction.

Common situations: Passing an object literal instead of an array; passing a single element instead of a list; mixing up objectContaining and arrayContaining; receiving data from an untyped source whose shape differs from expectation.

Related errors

AI-assisted analysis of jestjs/jest@8e6d128e4a (2026-08-10). Data as JSON: /api/errors/71fd3635eef2f006. Report an issue: GitHub.

Appendix: source

Thrown at packages/expect/src/asymmetricMatchers.ts:200

  toString() {
    return 'Anything';
  }

  // No getExpectedType method, because it matches either null or undefined.

  override toAsymmetricMatcher() {
    return 'Anything';
  }
}

class ArrayContaining extends AsymmetricMatcher<Array<unknown>> {
  constructor(sample: Array<unknown>, inverse = false) {
    super(sample, inverse);
  }

  asymmetricMatch(other: unknown) {
    if (!Array.isArray(this.sample)) {
      throw new TypeError(
        `You must provide an array to ${this.toString()}, not '${typeof this
          .sample}'.`,
      );
    }

    const matcherContext = this.getMatcherContext();
    const result =
      this.sample.length === 0 ||
      (Array.isArray(other) &&
        this.sample.every(item =>
          other.some(another =>
            equals(item, another, matcherContext.customTesters),
          ),
        ));

    return this.inverse ? !result : result;
  }

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