Stub function with return function with sinon?

Question

I want to make unit testing and cover my code, this is my code, how can cover the createClient with sinon ?

const client = redis.createClient({
  retry_strategy: function(options) {
    if (options.error) {
      if (options.error.code === 'ECONNREFUSED') {
        return new Error('The server refused the connection');
      }
      if (options.error.code === 'ECONNRESET') {
        return new Error('The server reset the connection');
      }
      if (options.error.code === 'ETIMEDOUT') {
        return new Error('The server timeouted the connection');
      }
    }
    if (options.total_retry_time > 1000 * 60 * 60) {
      return new Error('Retry time exhausted');
    }
    if (options.attempt > 10) {
      return undefined;
    }
    return Math.min(options.attempt * 100, 3000);
  }

Answer 1

The easiest way to test the function assigned to retry_strategy would be to move it outside the redis.createClient call and export it:

export const retryStrategy = function (options) {
  if (options.error) {
    if (options.error.code === 'ECONNREFUSED') {
      return new Error('The server refused the connection');
    }
    if (options.error.code === 'ECONNRESET') {
      return new Error('The server reset the connection');
    }
    if (options.error.code === 'ETIMEDOUT') {
      return new Error('The server timeouted the connection');
    }
  }
  if (options.total_retry_time > 1000 * 60 * 60) {
    return new Error('Retry time exhausted');
  }
  if (options.attempt > 10) {
    return undefined;
  }
  return Math.min(options.attempt * 100, 3000);
}

const client = redis.createClient({
  retry_strategy: retryStrategy
  ...

Then you can import it and test it directly:

import { retryStrategy } from './your-module';

test('retryStrategy', () => {
  expect(retryStrategy({ attempt: 5 })).toBe(500);  // SUCCESS
  ...
})