Why is the `message` for my Jest custom matcher not being displayed? - jestjs

Question

I've created a Jest custom matcher. It works (meaning, it passes/fails when it should), but I don't see the message anywhere in Jest's output.
What am I doing wrong? Do I have to do something to "enable" messages? Am I totally misunderstanding where the message is supposed to show up?
Environment: NestJS, Prisma
Execution command: jest --watch
Simplified code:
declare global {
namespace jest {
interface Matchers<R> {
toMatchHash(received: string, expected: string): R;
}
}
}
expect.extend({
toMatchJsonHash(received, expected) {
return {
pass: false,
message: () => `Why doesn't this work?!`,
};
},
});
expect(prisma.name.findMany).toHaveBeenCalledWith(expect.toMatchJsonHash('db0110285c148c77943f996a17cbaf27'));
Output:
● MyService › should pass a test using a custom matcher
expect(jest.fn()).toHaveBeenCalledWith(...expected)
Expected: toMatchJsonHash<db0110285c148c77943f996a17cbaf27>
Received: {<Big ol' object redacted for conciseness>}
Number of calls: 1
178 |
179 | // #ts-ignore
> 180 | expect(prisma.name.findMany).toHaveBeenCalledWith(expect.toMatchJsonHash('db0110285c148c77943f996a17cbaf27'));
| ^
181 | // expect(prisma.name.findMany).toHaveBeenCalledWith({
182 | // select: { type: true, name: true },
183 | // where: {
at Object.<anonymous> (my/my.service.spec.ts:180:32)
I'm expecting to see "Why doesn't this work?!" somewhere in the output, but I don't. What am I missing?

Answer 1

As suggested by #jonsharpe, the reason was that Jest was showing the message from the "outer" matcher, .toHaveBeenCalledWith().
To fix this, I found the source that defines the .toHaveBeenCalledWith() matcher and "merged" its code into my custom matcher.
This enabled my custom matcher to effectively "extend" the functionality of the .toHaveBeenCalledWith() matcher, including my own custom code and messages.
In case it helps someone, the code I ended up with for my specific use case was:
declare global {
namespace jest {
interface Matchers<R> {
toHaveBeenCalledWithObjectMatchingHash(expected: string): CustomMatcherResult;
}
}
}
expect.extend({toHaveBeenCalledWithObjectMatchingHash(received, expected) {
const isSpy = (received: any) =>
received != null &&
received.calls != null &&
typeof received.calls.all === 'function' &&
typeof received.calls.count === 'function';
const receivedIsSpy = isSpy(received);
const receivedName = receivedIsSpy ? 'spy' : received.getMockName();
const calls = receivedIsSpy
? received.calls.all().map((x: any) => x.args)
: received.mock.calls;
if(calls.length === 0) {
return {
pass: false,
message: () => `expected the function to be called with an object that hashes to '${expected}'. Instead, the function was not called.`,
};
}
if(calls[0].length === 0) {
return {
pass: false,
message: () => `expected the function to be called with an object that hashes to '${expected}'. Instead, the function was called, but not with any arguments.`,
};
}
const md5Hash = crypto.createHash('md5');
const receivedHash = md5Hash.update(JSON.stringify(calls[0][0])).digest('hex');
const pass = receivedHash === expected;
if(pass) {
return {
pass: true,
message: () => `expected the function to not be called with an object that hashes to '${expected}'. Instead, the passed object hashes to the same value.`,
};
} else {
return {
pass: false,
message: () => `expected the function to be called with an object that hashes to '${expected}'. Instead, the passed object hashes to '${receivedHash}'.`,
};
}
}});