vitest-dev/vitest · 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's asymmetricMatch throws a TypeError if this.sample is not an array. Like objectContaining, this fires at match time when the matcher is evaluated, not at construction. An empty array passes against any array (vacuous containment).

Solutions

1. Wrap the expected items in an array: expect.arrayContaining([1, 2, 3]).
2. Spread iterable sources: expect.arrayContaining([...setOfIds]).
3. For a single item, still use an array literal of length 1.

Example fix

// before
expect(ids).toEqual(expect.arrayContaining(42))
// after
expect(ids).toEqual(expect.arrayContaining([42, 43]))

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

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

When it happens

Trigger: expect.arrayContaining(sample) built with a non-array sample, then used in toEqual/toEqual-like assertions.

Common situations: Passing a single value instead of an array (arrayContaining(1) instead of arrayContaining([1])); passing a Set (use [...set]); passing an object where a list was intended.

Related errors

• any() expects to be passed a constructor function. Please…
• Expected is not a Number
• Expected is not a string
• Expected is not a String or a RegExp
• Precision is not a Number

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/da3fa524a506af87. Report an issue: GitHub.

Appendix: source

Thrown at packages/expect/src/jest-asymmetric-matchers.ts:201

  }

  toString() {
    return `Object${this.inverse ? 'Not' : ''}Containing`
  }

  getExpectedType() {
    return 'object'
  }
}

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

  asymmetricMatch(other: Array<T>, customTesters?: Array<Tester>): boolean {
    if (!Array.isArray(this.sample)) {
      throw new TypeError(
        `You must provide an array to ${this.toString()}, not '${typeof this
          .sample}'.`,
      )
    }

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

    return this.inverse ? !result : result
  }

  toString() {

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