vitest-dev/vitest · error · TypeError

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

Error message

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

What it means

Thrown as a TypeError inside ArrayContaining.asymmetricMatch when this.sample is not an array. ArrayContaining asserts the received value contains all expected elements, so the expected sample must be an array.

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() {

View on GitHub (pinned to d568f8ce37)

Solutions

1. Wrap the elements in an array: expect.arrayContaining([1, 2]).
2. Spread an iterable into an array: expect.arrayContaining([...set]).
3. Use objectContaining or stringContaining for non-array semantics.

Example fix

// before
expect(items).toEqual(expect.arrayContaining('alpha'))
// after
expect(items).toEqual(expect.arrayContaining(['alpha']))

function asArray<T>(v: unknown): T[] {
  if (!Array.isArray(v)) throw new TypeError(`arrayContaining needs an array, got ${typeof v}`)
  return v as T[]
}
expect(items).toEqual(expect.arrayContaining(asArray(expected)))

function isArrayOf<T>(v: unknown): v is T[] {
  return Array.isArray(v)
}

When it happens

Trigger: Constructing expect.arrayContaining(value) where value is not an array (a single item, a Set, a plain object). The constructor stores it but matching throws on Array.isArray(this.sample) === false.

Common situations: Passing a single element instead of an array; passing a Set/Map where an array was expected; refactor changing the shape of the expected value.

Related errors

• Expected is not a string
• You must provide an object to ${this.toString()}, not '${typ
• any() expects to be passed a constructor function. Please pa
• Expected is not a String or a RegExp
• Expected is not a Number

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/da3fa524a506af87.json. Report an issue: GitHub.