How to mock function call result with Mocha & Chai?

createServer.js

const express = require("express");
const mongoConnector = require("./mongoConnector");
const sequelizeConnector = require("./sequelizeConnector");
const {
    logger: { logger }
} = require("custom-error-exceptions");

class CreateServer {
  constructor(param = {}) {
    this.port = param.port || 3000;
    this.config = param.config || null;
    this.models = param.models;
    this.app = express();
    this.server;
  }

  choosingDatabase() {
    if (this.config && this.config.dbConfig) {
      const {
        dbConfig: { dialect },
      } = this.config;
      if (databases.sequelizeSupportDialect.includes(dialect)) {
        logger.info(`Using sequelize dialect ${dialect} database`);
        sequelizeConnector(this.app, this.config, logger, this.models);
      } else if (dialect === databases.mongodbDialect) {
        logger.info("Using mongodb database");
        const createMiddleware = mongoConnector(
          this.app,
          this.config,
          logger,
          this.models
        );
        this.app.use(createMiddleware);
      } else {
        logger.error("Dialect you're choosing ins'nt supported");
      }
    } else {
      logger.info("No database config found");
    }
  }
}

How to Mocking returned result function call sequelizeConnector & mongoConnector? I've finished a few lines of code on the choosingDatabase function, but when I do the same on the if (databases.sequelizeSupportDialect.includes(dialect) and else if (dialect === databases.mongodbDialect) blocks it actually calls real connection.how to mock the two functions? i hope someone can help me.

const chai = require("chai");
const sinon = require("sinon");
const sinonChai = require("sinon-chai");

const CreateServer = require("./createServer");

const { expect } = chai;
chai.should();
chai.use(sinonChai);

describe("CreateServer", () => {
  describe("choosingDatabase", () => {
    it("should return a function that calls createRouter", () => {
      const server = new CreateServer({});
      sinon.spy(server, "choosingDatabase");
      server.choosingDatabase();
      expect(server.choosingDatabase).calledOnce;
    });

    it("should return a function that calls createRouter", () => {
      const server = new CreateServer({ config: {} });
      sinon.spy(server, "choosingDatabase");
      server.choosingDatabase();
      expect(server.choosingDatabase).calledOnce;
    });

    it("should return a function that calls createRouter", () => {
      const server = new CreateServer({
        config: { dbConfig: { dialect: "wrong" } },
      });
      sinon.spy(server, "choosingDatabase");
      server.choosingDatabase();
      expect(server.choosingDatabase).calledOnce;
    });
  });
})

How to Mocking returned result function call sequelizeConnector & mongoConnector? I've finished a few lines of code on the choosingDatabase function, but when I do the same on the if (databases.sequelizeSupportDialect.includes(dialect) and else if (dialect === databases.mongodbDialect) blocks it actually calls real connection.how to mock the two functions? i hope someone can help me.