Why aren't jest expect functions available inside toMatchObject? - jestjs

Question

For example:
duty.forEach((dutyItem) => {
expect(dutyItem).toMatchObject({
person_id: expect.any(Number),
date: expect.any(String),
uxdate: expect.any(Number),
is_split: expect.any(Number),
split_rest_hours: expect.any(Number),
aircraft_type_id: expect.toMatch(new RegExp('[0-9]|null')),
type: expect.any(String),
status: expect.any(String),
location_start: expect.any(String),
start: expect.any(String),
uxstart: expect.any(Number),
location_end: expect.any(String),
end: expect.any(String),
uxend: expect.any(Number),
duty_time: expect.any(Number),
is_zulu: expect.any(Number),
is_flight: expect.any(Number),
});
});
returns:
TypeError: expect.toMatch is not a function
Explicitly matching the property like:
aircraft_type_id: expect(dutyItem.aircraft_type_id).toMatch(..) doesn't work either.

Answer 1

Look at the expect methods here. Note that there is no expect.toMatch(). The .toMatch() is a matcher function that can be used on the return value of expect(value) as in expect('abc').toMatch('abc').
It looks like what you're trying to do is say for each dutyItem the aircraft_type_id matches a certain regex. But you're trying to use a matcher as a value in an object and that's not quite right. Instead you can use expect.any(Number) (or String if it can be a string) and have a second more explicit expect statement like this:
duty.forEach((dutyItem)w => {
const airVal = dutyItem.aircraft_type_id === null ? null : expect.any(Number);
expect(dutyItem).toMatchObject({
person_id: expect.any(Number),
date: expect.any(String),
uxdate: expect.any(Number),
is_split: expect.any(Number),
split_rest_hours: expect.any(Number),
aircraft_type_id: airVal,
type: expect.any(String),
status: expect.any(String),
location_start: expect.any(String),
start: expect.any(String),
uxstart: expect.any(Number),
location_end: expect.any(String),
end: expect.any(String),
uxend: expect.any(Number),
duty_time: expect.any(Number),
is_zulu: expect.any(Number),
is_flight: expect.any(Number),
});
});
This allows the first match to check the structure of the whole dutyItem object, then do a more explicit test on the aircraft_type_id after.
A bit more detail on why what you tried didn't work... Consider the object you're asking .toMatchObject to compare with. If your code looks like this:
const matchObj = {
key: expect.any(String)
};
expect(obj).toMatchObject(matchObj);
This means you expect obj.key to match any string. But what is the value of matchObj.key? It's not a function, it's a special object that Jest knows means "any string" and when it's comparing the actual value of obj.key it knows to just check it against any string.
If instead you had this (if it worked) then what would matchObj.key be? It would be the return value of .toMatch(), which is probably undefined but in any case isn't a value that is useful during the .toMatchObject() function.
const matchObj = {
key: expect.toMatch(regex) // key will be the return value of .toMatch
};
expect(obj).toMatchObject(matchObj);