The callsFake() Function in Sinon – onlinecode

Sinon stubs have a callsFake() function that tells Sinon what function to call instead of the stubbed function.
For example, you can replace the axios.get() function with a fake function as follows.

const axios = require('axios');
const assert = require('assert');

const stub = sinon.stub(axios, 'get').
  callsFake(() => Promise.resolve({ status: 200 }));
// Calls the fake 'axios.get()'
const test = await axios.get('https://httpbin.org/get');

test.status; // 200
test.data; // undefined

The callsFake() function is handy for testing, because you can configure the behavior of any function call to handle hard-to-test code paths.

For Async Functions for The callsFake() Function in Sinon

To stub async functions, we typically recommend making your fake function return a promise using Promise.resolve().
Making sure your fake function returns a promise is especially important if you are using promise chaining, because otherwise you won’t be able to call then().

const stub = sinon.stub(axios, 'get').
  callsFake(() => Promise.resolve({ status: 200 }));

You can also pass an async function to callsFake().
However, that can be indicative of a code smell, because fake functions typically should not do anything besides returning a pre-defined value.
Multi-line fake functions are typically unnecessary.
However, you may use the following syntax instead of Promise.resolve() because the following is more concise.

const stub = sinon.stub(axios, 'get').
  callsFake(async () => ({ status: 200 }));

For Errors for The callsFake() Function in Sinon

You can also make your fake functions throw errors for testing error cases. For example, you can make a stub return a rejected promise as follows.

const axios = require('axios');
const assert = require('assert');

const stub = sinon.stub(axios, 'get').
  callsFake(() => Promise.reject(new Error('Oops!')));

// Calls the fake 'axios.get()'
try {
  await axios.get('https://httpbin.org/get');
} catch (err) {
  err.message; // Oops!
}

You can also throw an error from the fake function body.
Just make sure you’re consistent about async functions vs sync functions: if you’re stubbing an async function, make sure you either return a promise or use an async fake function!

// Good: fake returns a promise
sinon.stub(axios, 'get').
  callsFake(() => Promise.reject(new Error('Oops!')));

// Good: fake is async
sinon.stub(axios, 'get').
  callsFake(async () => { throw new Error('Oops!'); });

// Bad: fake throws sync error, even though axios.get() never does
sinon.stub(axios, 'get').
  callsFake(() => { throw new Error('Oops!'); });

Sinon is a JavaScript library that provides test spies, stubs, and mocks. It’s very flexible and easy to use since you can combine it with any testing framework.