How to force a function to throw exception when it invoked with Mocha/Chai

Question

I want to test the function B to catch exception thrown from function A with Mocha/Chai .

function A() {
  // 1. the third party API is called here
  // some exception may be thrown from it
  ...
  // 2. some exception could be thrown here
  // caused by the logic in this function
}

function B() {
  // To catch exception thrown by A()
  try {
     A();
  } catch(err) {
     console.error(err);
  }
  ...
}

I want force A to throw exception, while doing the test of B . So I can make sure the function B catch the exception from A correctly.

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After searching some posts:

Test for expected failure in Mocha

Testing JS exceptions with Mocha/Chai

I did not find the correct answer.

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Is my question reasonable? if it is, how to do that test with Mocha/Chai ?

Answer 1

This is called mocking. For the sake of test of function B you should mock function A to behave the proper way. So before the test you define A something like A = function(){ throw new Error('for test');} call and verify that when called B behaves accordingly.

describe('alphabet', function(){
    describe('A', function(){
         var _A;
         beforeEach(function(){
             var _A = A; //save original function
             A = function () {
                  throw new Error('for test');
             }
         });
         it('should catch exceptions in third party', function(){
             B();
             expect(whatever).to.be.true;
         });
         afterEach(function(){
             A = _A;//restore original function for other tests
         });
    }
})

As you're already using Mocha with Chai you might be interested to look into Sinon. It greatly extends Mocha capabilities. In this case you would use stubs which simplifies mocking and restoring