Suppose there is a class called Person which creates an instance of another class named Logger. How can we ensure that the method of Logger is being called when an instance of Person is created, as shown in the example below?
// Logger.ts
export default class Logger {
constructor() {}
log(m: String) {
console.log(m);
// Other operations that are outside testing (e.g., file write).
throw Error('error');
}
}
// Person.ts
import Logger from "./Logger";
export default class Person {
constructor() {
const logger = new Logger();
logger.log('created');
}
// ...
}
// Person.test.ts
import Person from "./Person";
import Logger from "./Logger";
describe('Person', () => {
it('calls Logger.log() on instantiation', () => {
const mockLogger = new Logger();
getCommitLinesMock = jest
.spyOn(mockLogger, 'log')
.mockImplementation(() => {});
new Person(); // Should call Logger.log() on instantiation.
expect(getCommitLinesMock).toBeCalled();
});
});
An alternative approach is to include Logger as a parameter in the constructor like so:
class Person {
constructor(logger: Logger) {
logger.log('created');
}
// ...
}
However, are there any other methods besides modifying the constructor function to pass this test successfully?