Expect positive number parameter passed - jest - node.js

Question

The test is linked to this question here which I raised (& was resolved) a few days ago. My current test is:
// Helpers
function getObjectStructure(runners) {
const backStake = runners.back.stake || expect.any(Number).toBeGreaterThan(0)
const layStake = runners.lay.stake || expect.any(Number).toBeGreaterThan(0)
return {
netProfits: {
back: expect.any(Number).toBeGreaterThan(0),
lay: expect.any(Number).toBeGreaterThan(0)
},
grossProfits: {
back: (runners.back.price - 1) * backStake,
lay: layStake
},
stakes: {
back: backStake,
lay: layStake
}
}
}
// Mock
const funcB = jest.fn(pairs => {
return pairs[0]
})
// Test
test('Should call `funcB` with correct object structure', () => {
const params = JSON.parse(fs.readFileSync(paramsPath, 'utf8'))
const { arb } = params
const result = funcA(75)
expect(result).toBeInstanceOf(Object)
expect(funcB).toHaveBeenCalledWith(
Array(3910).fill(
expect.objectContaining(
getObjectStructure(arb.runners)
)
)
)
})
The object structure of arb.runners is this:
{
"back": {
"stake": 123,
"price": 1.23
},
"lay": {
"stake": 456,
"price": 4.56
}
}
There are many different tests around this function mainly dependent upon the argument that is passed into funcA. For this example, it's 75. There's a different length of array that is passed to funcB dependent upon this parameter. However, it's now also dependent on whether the runners (back and/or lay) have existing stake properties for them. I have a beforeAll in each test which manipulates the arb in the file where I hold the params. Hence, that's why the input for the runners is different every time. An outline of what I'm trying to achieve is:
Measure the array passed into funcB is of correct length
Measure the objects within the array are of the correct structure:
2.1 If there are stakes with the runners, that's fine & the test is straight forward
2.2 If not stakes are with the runners, I need to test that; netProfits, grossProfits, & stakes properties all have positive Numbers
2.2 is the one I'm struggling with. If I try with my attempt below, the test fails with the following error:
TypeError: expect.any(...).toBeGreaterThan is not a function

Answer 1

As with previous question, the problem is that expect.any(Number).toBeGreaterThan(0) is incorrect because expect.any(...) is not an assertion and doesn't have matcher methods. The result of expect.any(...) is just a special value that is recognized by Jest equality matchers. It cannot be used in an expression like (runners.back.price - 1) * backStake.
If the intention is to extend equality matcher with custom behaviour, this is the case for custom matcher. Since spy matchers use built-in equality matcher anyway, spy arguments need to be asserted explicitly with custom matcher.
Otherwise additional restrictions should be asserted manually. It should be:
function getObjectStructure() {
return {
netProfits: {
back: expect.any(Number),
lay: expect.any(Number)
},
grossProfits: {
back: expect.any(Number),
lay: expect.any(Number)
},
stakes: {
back: expect.any(Number),
lay: expect.any(Number)
}
}
}
and
expect(result).toBeInstanceOf(Object)
expect(funcB).toHaveBeenCalledTimes(1);
expect(funcB).toHaveBeenCalledWith(
Array(3910).fill(
expect.objectContaining(
getObjectStructure()
)
)
)
const funcBArg = funcB.mock.calls[0][0];
const nonPositiveNetProfitsBack = funcBArg
.map(({ netProfits: { back } }, i) => [i, back])
.filter(([, val] => !(val > 0))
.map(([i, val] => `${netProfits:back:${i}:${val}`);
expect(nonPositiveNetProfitsBack).toEqual([]);
const nonPositiveNetProfitsLay = ...
Where !(val > 0) is necessary to detect NaN. Without custom matcher failed assertion won't result in meaningful message but an index and nonPositiveNetProfitsBack temporary variable name can give enough feedback to spot the problem. An array can be additionally remapped to contain meaningful values like a string and occupy less space in errors.