How to mock variables in nodejs - node.js

Question

When writing nodejs unit test case I want to mock variables.
let abc = awsMock.service.lambda;
In above code, awsMock I am creating and passing to method but service is json object which I want to mock. How I can pass some dummy value when above line is executed?
I want awsMock to be mock object only as I want to mock method using this after above code is executed.
I tried to stub variable but it didn't work

Answer 1

I recommend you encapsulate that clases who you need to mock in a service container and you can replace that values in your mock files.
I will use a minimal example case with three files:
- sc.js : The service container
- main.js: The main function
- test.js: The main function for tests
sc.js
// FAKE CLASS
class Lambda {
foo() {
return "real";
}
}
// END FAKE CLASS
// CODE START HERE
// The service container: could be an object, class, etc...
const serviceContainer = {
lambda: new Lambda(),
};
// Export SC
module.exports.sc = serviceContainer;
// Export properties with a wrap function (this is more easy to handle)
module.exports.getLambda = () => serviceContainer.lambda;
main.js
const { getLambda } = require("./sc");
module.exports.main = () => {
// Get lambda
const lambda = getLambda();
// Print foo()
console.log(lambda.foo());
}
test.js
const { sc } = require("./sc");
const { main } = require("./main");
// Exec before mock
main();
// Mock replace
sc.lambda = new class LambdaMock {
foo() {
return "mock";
}
}
// Exec after mock
main();
If you execute node test.js the output will be:
real
mock
So, you could use the getLambda() function in each file to get the Lambda Instance or the Mocked Lambda Instance.
NOTE: As a recommendation, you must wrap the AWS Lambda class in another class to make ensure the real and the mocked version will have the same functions.