vitest-dev/vitest · error · Error

Expected is not a string

Error message

Expected is not a string

What it means

Thrown by the StringContaining asymmetric matcher constructor when the sample is not a string (checked via Object.prototype.toString). StringContaining tests substring inclusion, so a non-string expected value is invalid at construction time.

Source

Thrown at packages/expect/src/jest-asymmetric-matchers.ts:77

}

// implement custom chai/loupe inspect for better AssertionError.message formatting
// https://github.com/chaijs/loupe/blob/9b8a6deabcd50adc056a64fb705896194710c5c6/src/index.ts#L29
// @ts-expect-error computed properties is not supported when isolatedDeclarations is enabled
// FIXME: https://github.com/microsoft/TypeScript/issues/61068
AsymmetricMatcher.prototype[Symbol.for('chai/inspect')] = function (options: { depth: number; truncate: number }): string {
  // minimal pretty-format with simple manual truncation
  const result = stringify(this, options.depth, { min: true })
  if (result.length <= options.truncate) {
    return result
  }
  return `${this.toString()}{…}`
}

export class StringContaining extends AsymmetricMatcher<string> {
  constructor(sample: string, inverse = false) {
    if (!isA('String', sample)) {
      throw new Error('Expected is not a string')
    }

    super(sample, inverse)
  }

  asymmetricMatch(other: string): boolean {
    const result = isA('String', other) && other.includes(this.sample)

    return this.inverse ? !result : result
  }

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

  getExpectedType() {
    return 'string'
  }

View on GitHub (pinned to d568f8ce37)

Solutions

1. Ensure the argument to expect.stringContaining is a string.
2. Coerce or stringify the value if substring semantics are intended.
3. For numeric/object checks, use the appropriate matcher (objectContaining, arrayContaining).

Example fix

// before
expect(obj).toEqual({ code: expect.stringContaining(404) })
// after
expect(obj).toEqual({ code: expect.stringContaining('NF') })

function asString(v: unknown): string {
  if (typeof v !== 'string') throw new TypeError(`stringContaining needs a string, got ${typeof v}`)
  return v
}
expect(obj).toEqual({ msg: expect.stringContaining(asString(sub)) })

function isString(v: unknown): v is string {
  return typeof v === 'string'
}

When it happens

Trigger: Constructing expect.stringContaining(value) where value is a number, object, null, etc. The isA('String', sample) guard fails and the constructor throws before any match.

Common situations: Passing a non-string by mistake (e.g. expect.stringContaining(123) or a variable typed as any that holds a number); refactoring where the expected value type changed.

Related errors

• You must provide an object to ${this.toString()}, not '${typ
• You must provide an array to ${this.toString()}, not '${type
• 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/ac82d9967a25e533.json. Report an issue: GitHub.