We can use Node.js directly for backend programming and there is no need to use Express.js. Actually, Express.js is a Node.js framework and it adds to the ease of programming and scalability of Node.js. We can also CODE the frontend in JavaScript, WITHOUT going through frameworks like React and Angular. But we rarely have a LARGE web-app in Vanilla JavaScript. 

Similarly, it’s not scalable to write the complete app in Node.js, because the code becomes complex very soon. So, we write the Node.js code with its framework of Express.js. The code for Express.js is very simple, minimalist, scalable, and can be learnt by a JavaScript developer in no time.

So, we don’t use Express.js over Node.js but with Node.js. We take the speed and performance of Node.js along with the simplicity of Express.js to create scalable and performant web-apps APIs. This is the reason, we never use Node.js without Express.js in production and they always go together. Node.js along with Express.js, is part of TWO popular stocks i.e., MEAN stack and MERN stack.

The growth of Node.js should be credited to Express.js also, which has made Node.js really easy to use. Just as bootstrap led to the SUCCESS of CSS, similarly Express.js led to the success of Node.js.