vitest-dev/vitest · error · TypeError

You must provide a Promise to expect() when using .resolves,

Error message

You must provide a Promise to expect() when using .resolves, not '${typeof obj}'.

What it means

TypeError thrown by the 'resolves' property getter when the value passed to expect() is not thenable (has no .then function). .resolves only makes sense for real Promise-like values; anything else is a programming error detected before any assertion runs.

Source

Thrown at packages/expect/src/jest-expect.ts:1086

  utils.addProperty(
    chai.Assertion.prototype,
    'resolves',
    function __VITEST_RESOLVES__(this: any) {
      const error = new Error('resolves')
      utils.flag(this, 'promise', 'resolves')
      utils.flag(this, 'error', error)
      const test: Test = utils.flag(this, 'vitest-test')
      const obj = utils.flag(this, 'object')

      if (utils.flag(this, 'poll')) {
        throw new SyntaxError(
          `expect.poll() is not supported in combination with .resolves`,
        )
      }

      if (typeof obj?.then !== 'function') {
        throw new TypeError(
          `You must provide a Promise to expect() when using .resolves, not '${typeof obj}'.`,
        )
      }

      const proxy: any = new Proxy(this, {
        get: (target, key, receiver) => {
          const result = Reflect.get(target, key, receiver)

          if (typeof result !== 'function') {
            return result instanceof chai.Assertion ? proxy : result
          }

          return (...args: any[]) => {
            utils.flag(this, '_name', key)
            const promise = Promise.resolve(obj).then(
              (value: any) => {
                utils.flag(this, 'object', value)
                return result.call(this, ...args)

View on GitHub (pinned to d568f8ce37)

Solutions

1. Ensure expect() receives a real promise: expect(asyncFn()).resolves or expect(promise).resolves.
2. If the value is synchronous, remove .resolves: expect(value).toBe(x).
3. Add the missing await/call so a promise is actually produced.

Example fix

// before
expect(getCount()).resolves.toBe(5); // getCount is sync
// after
expect(getCount()).toBe(5);

if (typeof (obj as any)?.then !== 'function') throw new TypeError(`expect() target is ${typeof obj}, not a Promise — drop .resolves`);

const isThenable = (v: unknown): v is PromiseLike<unknown> =>
  v != null && typeof (v as any).then === 'function';

When it happens

Trigger: expect(42).resolves.toBe(42); expect('foo').resolves; expect(undefined).resolves; expect({}).resolves. Triggered when typeof obj?.then !== 'function' at line 1085.

Common situations: The developer omitted await or forgot to call the async function: expect(getValue()).resolves where getValue returns a plain value; or asserting .resolves on a synchronous return.

Related errors

• You must provide a Promise to expect() when using .rejects,
• expected promise to throw an error, but it didn't
• .toMatch() expects to receive a string, but got ${typeof act
• toContain() expected a DOM node as the argument, but got ${t
• ${utils.inspect(assertion._obj)} is not a spy or a call to a

AI-assisted analysis of vitest-dev/vitest@d568f8ce37 (2026-08-03). Data as JSON: /data/errors/f845403b93357c70.json. Report an issue: GitHub.