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tags: curriculum

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# An introduction to express.js, frameworks, libraries and abstraction.

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### What is express?

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- Express.js is a minimal, unopinionated web framework for node.

- builds upon node's features, and offers a lot of tools that makes writing node apps MUCH easier.

- Implements a lot of low level functionality behind the scenes to take care of often repeated operations of a web server(writing routes, serving static assets, cookie parsing, POST parsing etc.)

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- for example, express has a form body parser, so you can focus on writing code to actually use your form data, rather than writing code to parse it (remember streams and chunking?)

- we'll learn more about express and how to use it later, but since we're now going to be using a framework, it's a good time to talk a little about frameworks and libraries in general.

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### A segue into the difference between a framework and a library

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- Both frameworks and libraries are code written by someone else that is used to help solve common problems.
- but they're not interchangeable terms.

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### Libraries


- A library is a set of reusable functions that can be usually be plugged in wherever you want.

- Libraries are written so that developers across the world dont have to rewrite the same bit of code from scratch.

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- You are in complete control of what the library does, you choose when and where to call a function from the library, it executes some code, and that ends your deal with the library.

- A library is a hired gun, it comes in, does exactly what you request, and leaves.

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### Frameworks

- A framework, however, is more like a blueprint for a process. There is a control flow already in place, a system of functions that do things in a certain way, in a certain order.

- The framework defines when and where you can insert your own code into this blueprint, i.e, it calls the code you plugged in, when it needs to.

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- Generally, frameworks work as one high level system, rather than a series of independent operations.

- If a framework is a building contractor, **they** have direct control over the workers, and processes used to build the house. You can provide your input to shape the house to your requirements, but they do the work.

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## Opinionated vs unopinionated frameworks

- Opinionated frameworks are those with opinions about the "right way" to handle any particular task.

- Unopinionated frameworks, by contrast, have far fewer restrictions on how to achieve a goal

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- Unopinionated frameworks make it easier for developers to use the optimal modules to complete a particular task, however at the cost that you need to find the modules yourself.

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- Express is unopinionated.

- You can configure the behaviour between a request and a response, with a large number of compatible modules (express calls these middleware). you can also make your own middleware.

- You can structure the express app in one file or multiple files, and using any directory structure.

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## Abstraction
- **People have been coding for a while.**

- **If something's hard, someone's probably done it before, and shared it for free.**

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- Abstraction is the process of hiding the more complex functionality of a program, and only giving the most relevant information to the programmer.

- Abstraction allows for code re-use, and libraries and frameworks are built on the idea of abstraction: they both abstract the underlying code and allow you to reuse functionality when you need it.

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## Express vs node - an example of abstraction

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To serve static assets (css,js,image) in node, you'd write a function that looks like this:

const handlePublic = (request, response, url) => {
  const extension = url.split('.')[1];
  const extensionType = {
    html: 'text/html',
    css: 'text/css',
    js: 'application/javascript',
    ico: 'image/x-icon',
  };

  // Replaced err with error in line 30
  const filePath = path.join(__dirname, '..', url);
  fs.readFile(filePath, (error, file) => {
    if (error) {
      console.log(error);
      response.writeHead(404, { 'Content-Type': 'text/html' });
      response.end('<h1>404 file not found</h1>');
    } else {
      response.writeHead(200, { 'Content-Type': extensionType[extension] });
      response.end(file);
    }
  });
};

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With express, you can serve a whole directory of static files (images, CSS, JavaScript etc.) from your express server with a single line of code.

app.use(express.static('public'));

This will automatically handle any requests for images, css and javascript files made from your site, and points to your public folder.

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So why teach node first?

**Because manually setting content-types and response http headers are fundamental to understanding how the web works.**

Frameworks are nice, but you shouldn't let them be a crutch.