使用Mocha测试功能内的JavaScript代码

Question

Supposing I have the following function inside an external javascript file sampleFunction.js:

function mathOperation(a,b){
    var sum = a + b;
    var diff = a - b;
    var product = a * b;
}

And I would want to perform unit tests on this function using mochaJS from another separate file, say test.js. In that file I want to test the mentioned function for different inputs, and test the results. For example, I want to check whether the sum is 30 and the difference is 10 if I pass (20,10) to the function.

So in my test file, the code would be like this:-

var expect = chai.expect;
describe('Testing the function mathOperation()',function(){
    //test cases for checking sum, difference and product in mathOperation()
    ...
    ...
});

How do I go about writing the test cases to check the values of the sum, difference etc?

The reason why I ask the question is to know how can we test much bigger functions written in javascript which do not return anything, and to know whether we can test and compare the values of the various variables inside the function.

Answer 1

how can we test much bigger functions written in javascript which do not return anything...

Unless that function updates / changes something external then it can't be verified. The only thing you can test is that the function completed successfully.

...and compare the values of the various variables inside the function

You can't, and this is nothing to do with unit testing or Mocha in particular, this is just plain old JavaScript rules. Variables declared within a function are scoped to that function only, they can't be accessed externally.