How to Use the Sinon stub() Function

Jan 3, 2021

With the stub() function, you can swap out a function for a fake version of that function with pre-determined behavior.
This is helpful for testing edge cases, like what happens when an HTTP request fails.

The sinon.stub() substitutes the real function and returns a stub object that you can configure using methods like callsFake().
Stubs also have a callCount property that tells you how many times the stub was called.
For example, the below code stubs out axios.get() for a function that always returns { status: 200 } and asserts that axios.get() was called once.

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

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

const test = await axios.get('https://httpbin.org/get');

assert.deepEqual(test, { status:200 }); 
assert.strictEqual(stub.callCount, 1); 

Using getCall()

Stubs also have a getCall() function that returns data on a particular function call.
For example, stub.getCall(0) returns an object that contains data on the first time the stub was called, including arguments and returnValue:

const call = stub.getCall(0);

call.args; 
call.returnValue;