Custom failure message for Jest test - jestjs

Question

I'm testing a function with partially randomized input. The pieces of the input object that are randomized should work no matter what but if I happen to get a case that fails I'd like to know what it is by including the invalid input in my failure message.
How can I get Jest to show a failure message that extends further than just the contents of expect?
Here is the code I have:
describe("my module", () => {
it("should pass with some randomized inputs", () => {
const partialRandomInput = {
name: "Hardcoded",
age: Math.random() * 100,
yearsOfExperience: Math.random() * 30
};
const actualOutput = myModule(partialRandomInput);
expect(actualOutput).toEqual(false); // If this fails I only see the expect comparison that happened, which may not be helpful
});
});
Perhaps in my code above, the only failure would be if the randomized age is less than 5 and the randomized yearsOfExperience is less than 10. I'd like to see the values used for partialRandomInput when my test fails.

Answer 1

Do not test with random input.
Even if the code is meant to be used with random number, you should test it with predefined values to ensure it's consistent results. By using random numbers you cannot ensure that the used input will cover the same "branches" within the implementation.
What you can try instead is have a predefined set of values and expected results
describe.each([
[10, 10, true],
[22, 10, false],
[35, 11, true],
[50, 3, false],
[60, 7, false],
])(
"with age: %p and yearsOfExperience: %p",
(age, yearsOfExperience, expected) => {
it(`should return: ${expected}`, () => {
expect(
myModule({
name: "Hardcoded",
age,
yearsOfExperience,
})
).toEqual(expected);
});
}
);
But in order to answer the question - you can generate the random numbers before the execution of the test and you can then either:
Extend expect
Add the value within the description of the test
describe.each(
Array.from(new Array(10), () => [Math.random() * 100, Math.random() * 10])
)("with age: %p and yearsOfExperience: %p", (age, yearsOfExperience) => {
test(`returns false`, () => {
expect(
myModule({
name: "Hardcoded",
age,
yearsOfExperience,
})
).toBe(false);
});
});