Class to be tested
import { Lambda } from 'aws-sdk';
export default class Service {
public async callLambda(): Promise<void> {
const lambda = new Lambda();
const params: Lambda.Types.InvocationRequest = {
FunctionName: `function-name`,
InvocationType: 'Event',
};
await lambda.invoke(params).promise();
}
}
- Let suppose that we have a service that call a lambda using the
aws-sdklibrary. -
aws-sdkversion:2.546.0.
Unit test with the mock
First way
import Service from '../../../../src/api/services/Service';
const fakePromise = {
promise: jest.fn(),
};
jest.mock('aws-sdk', () => ({
Lambda: jest.fn(() => ({
invoke: () => fakePromise,
})),
}));
describe('callLambda', () => {
it('should return something... ', async done => {
const service = new Service();
const result = await service.callLambda();
expect(result).toBeUndefined();
done();
});
it('should throw an error... ', async done => {
// modifying the implementation before call the function
fakePromise.promise = jest.fn()
.mockImplementation(() => Promise.reject(new Error()));
try {
const service = new Service();
const result = await service.callLambda();
expect(result).toBeUndefined();
} catch (error) {
expect(error).toBeDefined();
}
done();
});
});
Second way
import { Lambda } from 'aws-sdk';
import Service from '../../../../src/api/services/Service';
// removing the factory function of the first way
jest.mock('aws-sdk');
describe('callLambda', () => {
// moving the fake to inside our describe test
// because we don't need it in jest.mock
const fakePromise = {
promise: jest.fn(),
};
beforeEach(() => {
// adding the implementation before each test
(Lambda as any).mockImplementation(() => {
return {
invoke: () => fakePromise,
};
});
});
it('should return something... ', async done => {
const service = new Service();
const result = await service.callLambda();
expect(result).toBeUndefined();
done();
});
it('should throw an error... ', async done => {
// modifying the implementation before call the function
fakePromise.promise = jest.fn()
.mockImplementation(() => Promise.reject(new Error()));
try {
const service = new Service();
const result = await service.callLambda();
expect(result).toBeUndefined();
} catch (error) {
expect(error).toBeDefined();
}
done();
});
});
- Inside the unit tests we can just change the
fakePromiseor update themockImplementationto pretend the behaviors that we need. - We can use these approaches to create unit test for other classes inside the
aws-sdk. -
jestversion:24.9.0.
Conclusion
The most difficult part of writing unit test is creating the mocks for external libraries, the purpose of this article is just help someone with troubles to mock this kind of library. We have a lot of ways to mock libraries, feel free to comment and send suggestions.
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