I'm trying to run this jestJS unit-test, but I do get TypeError: Promise resolver undefined is not a function. What am I doing wrong?

it('_onSubmit() should throw error if data is missing', (done) => {
  const createUserMutation = () => new Promise()
  const wrapper = shallow(<CreateAccount createUserMutation={createUserMutation} />)
  wrapper.update().find(Form).simulate('submit', {
    preventDefault: () => {}
  })
  createUserMutation.resolve().then(() => {
    expect(console.error).toHaveBeenCalled()
    done()
  })
})

I'm trying to run this jestJS unit-test, but I do get TypeError: Promise resolver undefined is not a function. What am I doing wrong?

it('_onSubmit() should throw error if data is missing', (done) => {
  const createUserMutation = () => new Promise()
  const wrapper = shallow(<CreateAccount createUserMutation={createUserMutation} />)
  wrapper.update().find(Form).simulate('submit', {
    preventDefault: () => {}
  })
  createUserMutation.resolve().then(() => {
    expect(console.error).toHaveBeenCalled()
    done()
  })
})

• javascript
• reactjs
• jestjs

• 3 The Promise constructor expects a function. And promise instances don’t have a resolve method. – Felix Kling Commented Nov 11, 2017 at 16:18
• 1 I'm having an extremely hard time trying to figure out how this test is supposed to work. Can you walk us through the thought process that led to this code and what you're trying to use the promise for? – JLRishe Commented Nov 11, 2017 at 16:30
• Why are you using promises here at all? What part of the code is asynchronous? – Bergi Commented Nov 11, 2017 at 16:30

3 Answers 3

new Promise() is trying to create a promise without an executor function*. The correct usage is to pass in a function that accepts up to two parameters (resolve and reject), e.g.:

var p = new Promise((resolve, reject) => {
    // ...code that does something, ultimately calls either resolve or reject
});

More on MDN:

• The Promise constructor
• Using Promises

Later, you appear to be trying to call resolve on the promise instance. You can't do that either (unless you're not using standard promises). You resolve a promise using the resolve function passed into the executor. (I tried to modify your code for you, but too much context info is required.)

* I'm surprised to see V8's error message say "resolver function" but I've verified it does. The spec calls it an executor, and I would have thought a "resolver functioN" would be the function the Promise constructor passes as the first argument to it...

If all you want to do is create a resolved promise, then use Promise.resolve(), which returns a new, already resolved promise.

new Promise() is for converting non-promise (usually asynchronous) code to promises, but it doesn't look like you need that here. If you were going to use it, you would have to pass a function into it (as Felix Kling and TJ Crowder have already noted).

Some more points:

• You are defining createUserMutation as a function, so a few lines later when you use it, you would need to call it using (). Or you could choose to have it not be a function since it seems all you're trying to do here is produce a promise.
• Also, as Felix Kling has pointed out, promises don't have a resolve() method. You can just call .then() on the promise produced by Promise.resolve().

In my case using types like TypeScript was an issue.

const a = new Promise<any>((resolve, reject) => {})

This wasn't acceptable in JavaScript and it spits the error "Promise resolver is not a function". However, when I deleted the type it works fine.

const a = new Promise((resolve, reject) => {})