Jest: toThrow() matcher - reactjs

Question

Context: React application with mobx.
Anyway I've a class (a store), catalogStore, with a loadProducts method. This method call a service to get the data and then, return it.
I've to write a test that say "If it cannot get the data, then throw an exception"
I mocked the function supposed to get the data, forcing it to reject... ok
This is the test I wrote
describe("catalogStore", () => {
describe("if the catalog fails to get the data", () => {
beforeAll(() => {
catalogService.get = jest.fn().mockImplementation(() => {
return new Promise((resolve, reject) => {
reject("rejected error");
});
});
});
it("should throw an error", () => {
return expect(() => catalogStore.loadProducts()).toThrow();
});
});
});
And this is the loadProducts function:
loadProducts() {
return catalogService
.get()
.then(result => {
this.products = result.services;
return {products: this.products};
})
.catch(error => {
console.log("CatalogStore loadProducts error catch: ", error);
return { error };
})
.then(({ error }) => {
if (error) {
console.log("Im gonna throw the error -> ", error);
throw error;
}
});
}
From the logs I can see "Im gonna throw the error -> rejected error", but the test fails with this message:
Expected the function to throw an error. But it didn't throw anything.
Why? I'm throwing the error.
Luca

Answer 1

Your error is thrown in the context of a Promise chain callback. It will be caught by the Promise and passed to the next catch handler.
To modify your test to inspect the error you could use Jest's Promise expectations:
describe("catalogStore", () => {
describe("if the catalog fails to get the data", () => {
beforeAll(() => {
catalogService.get = jest.fn().mockImplementation(() => {
return new Promise((resolve, reject) => {
reject("rejected error");
});
});
});
it("should throw an error", () => {
return expect(catalogStore.loadProducts()).rejects.toThrow('rejected error');
});
});
});

Answer 2

Its cause the function returns a promise, so all jest see is that the get() function is the called but as the error happens in a promise the test is finished before the error is thrown. To test that promises have look on how async error handling works.
Main idea is that you have an async function where you catch the failing promises by yourself
it('fails', async()=>{
try{
await catalogStore.loadProducts()
} catch(e) {
expect(e).toBeDefined()
}
})