As of now jest matcher toThrow help us to match only the Exception class or the error message.
If we want to do any matching against the custom details on the error instance, we have to use try...catch and assign the error to a variable and do expect manually like below
let err: ValidationError;
try {
await validatorDto(/* params */);
} catch (error) {
err = error;
}
expect(err.details).toEqual(
expect.arrayContaining([
expect.stringMatching(/currencycode must be a valid/i),
]),
);
In this blog, I will explain on how we can create a custom matcher to make this simple
First we will start with creating a setupAfterEnv.ts file and add it to setupFilesAfterEnv in jest config.
// jest.config.js or package.json
{
// other configs
"setupFilesAfterEnv": [
"<rootDir>/test/setupAfterEnv.ts"
]
}
You can make changes to the paths based on your jest configuration of rootDir and path to your setupAfterEnv.ts file.
Now let’s use expect.extend to add our custom matcher method.
// test/setupAfterEnv.ts
expect.extend({
async toThrowWithErrorMessage(received, expected) {
let err: { details: string[] };
try {
await received();
} catch (error) {
err = error;
}
if (this.isNot) {
expect(err.details).not.toEqual(
expect.arrayContaining([expect.stringMatching(expected)]),
);
} else {
expect(err.details).toEqual(
expect.arrayContaining([expect.stringMatching(expected)]),
);
}
return {
pass: !this.isNot,
message: () => '',
};
},
});
In the above custom matcher, we assume that all our custom Error instances will hold the messages in the .details property.
Next, let’s add types so the typescript won’t complain about your new custom matcher.
// index.d.ts
declare namespace jest {
interface Matchers<R> {
toThrowWithErrorMessage: (received: any) => void;
}
interface Expect {
toThrowWithErrorMessage: (received: any) => void;
}
}
Here is how we use the new custom matcher,
it('should throw error with error message', async () => {
return expect(async () => {
await validatorDto(Dto, {});
}).toThrowWithErrorMessage(/name should not be empty/i);
});
Hope that Helped.
if you have any suggestions or improvement, please drop a line to me over email
Versions of Language/packages used in this post.
| Library/Language | Version |
| ---------------- |---------|
| Jest | 28.1.2 |
| TypeScript | 4.9.5 |