Why does jest mockResolvedValueOnce called multiple times returns the same value? - node.js

Question

I have a class method where I trigger a #google-cloud/firestore multiple times. I would like to mock the call over the same .get() method multiple times.
Using a mockResolvedValueOnce multiple times with different values to return, the 2nd value is ignored.
jest.doMock('#google-cloud/firestore', () => class {
collection () {
return {
get: jest.fn().mockResolvedValue({
docs: []
}),
doc: () => {
return {
set: jest.fn(),
get: jest.fn().mockResolvedValueOnce({})
}
},
limit: () => {
return {
get: jest.fn().mockResolvedValue({ empty: true })
}
},
onSnapshot: jest.fn(),
select: () => {
return {
get: jest.fn() // <------------ MULTIPLE CALLS CHAINED BELOW
.mockResolvedValueOnce({
size: 1
}).mockResolvedValueOnce({
size: 2
})
}
}
}
}
})
When I console.log(snapshot.size) it returns me the same value "1" twice for both calls.
if (isEmptyModels || isStatsEmptyModels) {
// ...
console.log('📊 [STATS][MODELS] - Fulfilling the counters')
await Database.collection('models').select('id').get().then(snapshot => {
console.log(snapshot.size) // <--------- 1st call
this.fields.models.count = snapshot.size
this.fields.models.linked = snapshot.size
})
// ...
}
if (isEmptyProducts1P || isStatsEmptyProducts1P) {
// ...
console.log('📊 [STATS][PRODUCTS1P] - Fulfilling the counters')
await Database.collection('products1P').select('isMaintained').get().then(snapshot => {
console.log(snapshot.size) // <--------- 2nd call
snapshot.forEach(doc => {
if (doc.data().isMaintained) {
// ...
}
})
// ...
})
// ...
}
Why is that, and what is done wrong here ?
Error message is:
console.log
📊 [STATS][MODELS] - Fulfilling the counters
at Statistics.fulfillProductsCount (app/services/statistics/index.js:95:15)
console.log
1
at app/services/statistics/index.js:97:17
console.log
📊 [STATS][PRODUCTS1P] - Fulfilling the counters
at Statistics.fulfillProductsCount (app/services/statistics/index.js:106:15)
console.log
1
at app/services/statistics/index.js:108:17
TypeError: snapshot.forEach is not a function
117 | await Database.collection('products1P').select('isMaintained').get().then(snapshot => {
118 | console.log(snapshot.size)
> 119 | snapshot.forEach(doc => {
| ^
120 | if (doc.data().isMaintained) {
121 | this.fields.products1P.maintained += 1
122 | } else {
at app/services/statistics/index.js:119:18

Answer 1

This happens because each time you call Database.collection(), it creates a new object, and as a new object, this is the first time its properties are called. It is also valid for the others functions inside collection.
What I mean is that Database.collection is a function that returns an object that contains other functions that return object that contains properties mocked. By mocking this way, you will never be able to use mock...ValueOnce. But, I see two ways to "bypass" this problem :
1 - The short but conflicting way
You can use .mockReturnThis() to avoid entering into deep mock objects/functions, but in may be quickly conflicting when dealing with "fat" classes that have multiple times the same method names. In can also be helpful when mocking chainable methods (example: ORM queries with .find().filter().sort()...).
jest.doMock('#google-cloud/firestore', () => class {
collection = jest.fn().mockReturnThis();
select = jest.fn().mockReturnThis();
get = jest.fn().mockResolvedValueOnce({ size: 1 }).mockResolvedValueOnce({ size: 2 });
})
2 - The Long but working way
Mock the whole collection method Once instead of mocking only collection().select().get().
Database.collection.prototype.mockReturnValueOnce({
select: () => {
get: () => ({ size: 1 })
}
}).mockReturnValueOnce({
select: () => {
get: () => ({ size: 2 })
}
})
--> You will need access to the mocked Class and mock the method "collection" of the prototype (collection = jest.fn()).