jukkatupamaki/jest-async-error.md

## jest-async-error.md

      
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              jest-async-error.md
            
          
    Asserting async errors with Jest

Question: How to test what kind of error an async function throws and how to apply matchers to the rejected value?
Answer: You need to await the expect call and then inspect the rejects property of the returned Jest object.
async function doSomethingAsync(value?: string) {
  if (!value) throw new MyFancyError('Some error text');
  return value;
}

const error = await expect(doSomethingAsync()).rejects;
It's still a Promise so you need to await the matchers, too.
await error.toBeInstanceOf(MyFancyError);
await error.toMatch(/Some error text/i);
await error.toHaveProperty('foo', 'bar');
Docs: https://jestjs.io/docs/expect#rejects