vitest-dev/vitest · error · TypeError

You must provide an array or set to

Error message

You must provide an array or set to ${matcherHint('.toBeOneOf')}, not '${typeof expected}'.

What it means

The toBeOneOf matcher requires its expected argument to be an Array or a Set; any other type raises a TypeError before equality is evaluated. An empty Array/Set passes by default (matches anything), but a primitive or plain object is rejected.

Solutions

Wrap the candidate value(s) in an array: toBeOneOf(['active']).
Use a Set when testing membership against a large or deduplicated collection.
If you meant a single value, use toBe(value) instead.

Example fix

// before
expect(status).toBeOneOf('active')
// after
expect(status).toBeOneOf(['active', 'idle', 'blocked'])

Defensive patterns

Strategy: type-guard

Validate before calling

function assertCandidates(v) {
  if (!Array.isArray(v) && !(v instanceof Set)) {
    throw new TypeError('toBeOneOf expects an Array or Set')
  }
}

Type guard

function isArrayOrSet(v): v is Array<unknown> | Set<unknown> {
  return Array.isArray(v) || v instanceof Set
}

Prevention

Always pass an array literal to toBeOneOf, even for a single value.
Use toBe(value) when only one value is acceptable.
Lint for toBeOneOf called with string/number literals.

When it happens

Trigger: expect(actual).toBeOneOf(expected) where expected is a string, number, object literal, Map, or any non-Array/non-Set value.

Common situations: Passing a single allowed value unwrapped (toBeOneOf('active') instead of toBeOneOf(['active'])); confusing toBeOneOf with toBe; passing a Map where a Set was intended.

Related errors

AI-assisted analysis of vitest-dev/vitest@1fa9837ec2 (2026-08-11). Data as JSON: /api/errors/3a01f052e9874f33. Report an issue: GitHub.

Appendix: source

Thrown at packages/expect/src/custom-matchers.ts:46

  },

  toBeOneOf(actual: unknown, expected: Array<unknown> | Set<unknown>) {
    const { equals, customTesters } = this
    const { printReceived, printExpected, matcherHint } = this.utils

    let pass: boolean

    if (Array.isArray(expected)) {
      pass = expected.length === 0
        || expected.some(item =>
          equals(item, actual, customTesters),
        )
    }
    else if (expected instanceof Set) {
      pass = expected.size === 0 || expected.has(actual) || [...expected].some(item => equals(item, actual, customTesters))
    }
    else {
      throw new TypeError(
        `You must provide an array or set to ${matcherHint('.toBeOneOf')}, not '${typeof expected}'.`,
      )
    }

    return {
      pass,
      message: () =>
        pass
          ? `\
${matcherHint('.not.toBeOneOf', 'received', '')}

Expected value to not be one of:
${printExpected(expected)}
Received:
${printReceived(actual)}`
          : `\
${matcherHint('.toBeOneOf', 'received', '')}

View on GitHub (pinned to 1fa9837ec2)