How in Mocha test function with console.log statement?

Question

Let's say, I have a function:

function consoleOutput(param) {
 var newParam = param * param; 
 console.log(newParam);
}

How can I test in Mocha, that this function will be working correctly (param will be multiplied by two and output to the console). Thanks.

Answer 1

A great library for these types of tests is Sinon. It can be used to "hook" existing functions and track how those functions get called.

For example:

const sinon = require('sinon');
const assert = require('assert');
// the function to test
function consoleOutput(param) {
 var newParam = param * param; 
 console.log(newParam);
}
it('should log the correct value to console', () => {
 // "spy" on `console.log()`
 let spy = sinon.spy(console, 'log');
 // call the function that needs to be tested
 consoleOutput(5);
 // assert that it was called with the correct value
 assert(spy.calledWith(25));
 // restore the original function
 spy.restore();
});

The advantage of this is that you don't need to change the original function (which, in this case, isn't a big deal, but may not always be possible in larger projects).

• Fela Maslen: I'm getting TypeError: Attempted to wrap log which is already wrapped on the sinon.spy line?
• robertklep: @srynznfyra are you sure you're calling .restore() on the spy after every test?
• Fela Maslen: Yes, I fixed it by replacing spy.restore() with console.log.restore().