Mocking runtime config value with sinon

Question

I am adding some config values before hapi server start. Application is works fine although in test I can not use config.get(). I can get around with proxyquire. So I was wondering

• Is adding config file "dynamically" is bad design?
• Is there a way I can use config.get() in such suitation?

• Any alternative approach?

  //initialize.js
  
  const config = require('config');
  
  async function startServe() {
    const someConfigVal = await callAPIToGetSomeJSONObject();
    config.dynamicValue = someConfigVal;
    server.start(); 
  }
  
  //doSomething.js
  
  const config = require('config');
  
  function doesWork() {
    const valFromConfig = config.dynamicValue.X;
    // In test I can use proxiquire by creating config object
    ...
  }
  
  function doesNotWork() {
    const valFromConfig = config.get('dynamicValue.X'); 
    // Does not work with sinon mocking as this value does not exist in config when test run.
    // sinon.stub(config, 'get').withArgs('dynamicValue.X').returns(someVal);
    .....
  }
  

Answer 1

Context: testing.

• Is adding config file "dynamically" is bad design? => No. I have done it before. The test code changes configuration file: default.json in mid test to check whether function under test behaves as expected. I used several config utilities.
• Is there a way I can use config.get() in such suitation? => Yes. For sinon usage, see the example below, which use mocha. You need to define the stub/mock before the function under test use it, and do not forget to restore the stub/mock. Also there is official documentation related to this: Altering configuration values for testing at runtime, but not using sinon.

const config = require('config');
const sinon = require('sinon');
const { expect } = require('chai');

// Example: simple function under test.
function other() {
  const valFromConfig = config.get('dynamicValue.X');
  return valFromConfig;
}

describe('Config', function () {
  it ('without stub or mock.', function () {
    // Config dynamicValue.X is not exist.
    // Expect to throw error.
    try {
      other();
      expect.fail('expect never get here');
    } catch (error) {
      expect(error.message).to.equal('Configuration property "dynamicValue.X" is not defined');
    }
  });

  it('get using stub.', function () {
    // Create stub.
    const stubConfigGet = sinon.stub(config, 'get');
    stubConfigGet.withArgs('dynamicValue.X').returns(false);
    // Call get.
    const test = other();
    // Validate te result.
    expect(test).to.equal(false);
    expect(stubConfigGet.calledOnce).to.equal(true);
    // Restore stub.
    stubConfigGet.restore();
  });

  it('get using mock.', function () {
    // Create mock.
    const mockConfig = sinon.mock(config);
    mockConfig.expects('get').once().withArgs('dynamicValue.X').returns(false);
    // Call get.
    const test = other();
    // Validate te result.
    expect(test).to.equal(false);
    // Restore mock.
    expect(mockConfig.verify()).to.equal(true);
  });
});

Hope this helps.