Test case fails for toThrow() matcher - unit-testing

Question

I am new to unit testing in JS, learning jest framework. I have problem with simple toThrow() matcher that I test my throwing error function with.
I have written simple foo() function that only throws an Error and test it with toThrow() matcher.
index.js
export const foo = () => {
throw new Error('bar');
};
index.test.js
import {foo} from './index';
test('foo', () => {
expect(foo()).toThrow();
});
As far as I understand, as the function throws error in any case, the expected result checked by toThrow() should resolve to a passed test. However, when I run yarn test I get the following failure:
FAIL index.test.js
✕ foo (3ms)
● foo
bar
21 |
22 | export const foo = () => {
> 23 | throw new Error('bar');
| ^
24 | };
25 |
at foo (index.js:23:9)
at Object.<anonymous> (index.test.js:13:10)
Test Suites: 1 failed, 1 total
Is there some error in my code or maybe my understanding of the toThrow() matcher?

Answer 1

You need to pass a function to expect when using toThrow.
In this case you can simply pass foo:
index.js
export const foo = () => {
throw new Error('bar');
};
index.test.js
import { foo } from './index';
test('foo', () => {
expect(foo).toThrow(); // Success!
});
If throwing depends on a parameter then you can pass an arrow function that calls your function:
index.js
export const foo = bar => {
if (bar > 0) throw new Error('bar > 0');
};
index.test.js
import { foo } from './index';
test('foo', () => {
expect(() => foo(0)).not.toThrow(); // Success!
expect(() => foo(1)).toThrow(); // Success!
});