Mocking runtime config value with sinon - config

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.