I have a simple function as below
function foo({ platform }) {
if (platform === 'all') {
throw new Error('Platform value can only be android or ios');
}
return `${platform}`;
}
Then I wrote unit testing as below
it('should return correct result with platform', () => {
expect(foo({ platform: 'ios' })).toBe('ios');
expect(foo({ platform: 'android' })).toBe('android');
expect(foo({platform: 'all'})).toThrow(new Error('Platform value can only be android or ios'));
});
The test actually failed due to last test case without any useful information
FAIL src/utils/__test__/foo.test.ts
● foo() › should return correct result with platform
Platform value can only be android or ios
16 | }) {
17 | if (platform === 'all') {
> 18 | throw new Error('Platform value can only be android or ios');
| ^
19 | }
20 |
21 | return `${platform}`;
at xx (src/utils/foo.ts:18:11)
I've also tried to wrapped the entire expect with try catch block but the test is not passing either
toThrow only works when expect is passed a no-arg function that can be called to cause the exception.
/* this is the only test that needs this */
expect(() => foo({platform: 'all'})).toThrow();
This is because JavaScript does not support auto-deferring function arguments so there's no way for expect to "pause" the evaluation of its arguments. So the API has to be "give me (Jest) a function to call and I'll call it when I'm ready to handle what that function might do".